Seagen Inc. v. Daiichi Sankyo Company, Ltd.

Case: 23-2424    Document: 72      Page: 1   Filed: 12/02/2025




   United States Court of Appeals
       for the Federal Circuit
                  ______________________

                      SEAGEN INC.,
                      Plaintiff-Appellee

                              v.

        DAIICHI SANKYO COMPANY, LTD.,
      ASTRAZENECA PHARMACEUTICALS LP,
            ASTRAZENECA UK LTD.,
               Defendants-Appellants
              ______________________

                   2023-2424, 2024-1176
                  ______________________

     Appeals from the United States District Court for the
 Eastern District of Texas in No. 2:20-cv-00337-JRG, Judge
 J. Rodney Gilstrap.
                  ______________________

                Decided: December 2, 2025
                 ______________________

    SARA TONNIES HORTON, Willkie Farr & Gallagher LLP,
 Chicago, IL, argued for plaintiff-appellee. Also represented
 by DEVON WESLEY EDWARDS, New York, NY; DANE
 SOWERS, Washington, DC; MATTHEW A. CHIVVIS, MATTHEW
 IAN KREEGER, Morrison & Foerster LLP, San Francisco,
 CA; SETH W. LLOYD, BRIAN ROBERT MATSUI, Washington,
 DC; BRYAN J. WILSON, Palo Alto, CA.

   CHRISTOPHER NEIL SIPES, Covington & Burling LLP,
 Washington, DC, argued for defendants-appellants. Also
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 2               SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




 represented by BRADLEY KEITH ERVIN, NICHOLAS LANE
 EVOY, ROBERT JASON FOWLER.
                ______________________

     Before LOURIE, REYNA, and CHEN, Circuit Judges.
 LOURIE, Circuit Judge.
      A jury in the United States District Court for the East-
 ern District of Texas found that claims 1–5, 9, and 10 of
 Seagen Inc.’s (“Seagen”) U.S. Patent 10,808,039 (“the ’039
 patent”) were not invalid for lack of written description or
 enablement. J.A. 57. The jury further found that Daichii
 Sankyo Company, Ltd. AstraZeneca Pharmaceuticals LP,
 and AstraZeneca UK Ltd., (collectively, “Daichii”) willfully
 infringed at least one of the claims, and awarded Seagen
 damages exceeding $41 million. J.A. 56, 58–59. The dis-
 trict court denied Daichii’s post-trial motion for judgment
 as a matter of law (“JMOL”) and entered final judgment in
 favor of Seagen. J.A. 32–52. Because we conclude that the
 district court erred in failing to grant JMOL for lack of
 written description and enablement, we reverse.
                        BACKGROUND
                               I
      This case concerns a type of cancer treatment known
 as an antibody-drug conjugate (“ADC”). An ADC generally
 consists of three components: (1) an antibody, (2) a cyto-
 toxic (cell-killing) drug, and (3) a “linker” protein, which
 connects the antibody and drug. J.A. 3689 (depicting a sim-
 plified ADC). A linker may include different subcompo-
 nents. See J.A. 3548. Most relevant to this appeal is one
 potential subcomponent called a “peptide unit,” which is
 made of a chain of amino acids. See J.A. 3107 (215:2–4).
 Peptide units that include two amino acids are called di-
 peptides, those with three amino acids are called tripep-
 tides, etc. J.A. 3144 (31:20–32:8). Another subcomponent
 that may be included in a linker protein is a “spacer unit,”
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.                 3



 which is the part of the linker that is directly connected to
 the drug moiety (the moiety is the active ingredient within
 the drug compound), when present. See J.A. 3548.
      Unlike traditional chemotherapies, which are rela-
 tively non-selective with respect to the cells they kill, by
 combining the cell-targeting abilities of antibodies with the
 cell-killing abilities of cytotoxic drugs, an ADC is designed
 to kill only cancer cells while sparing healthy cells. J.A.
 3139–40 (11:16–13:3). One way in which an ADC functions
 is through a mechanism called “intracellular cleavage.” In
 principle, when an ADC administered to a patient is de-
 signed to intracellularly cleave, the antibody recognizes
 and targets specific cells, bringing along with it the linker
 and drug moiety. J.A. 3139 (12:17–23). Then, once the
 ADC reaches the target cell, it is absorbed into the cell, and
 the linker is cleaved (severed) from the antibody, releasing
 the drug moiety that kills the target cancer cell. J.A. 3141
 (18:2–19:22).
     Whether and how a drug moiety is cleaved from the
 ADC is determined, in part, by the components of the
 linker protein. J.A. 3140 (13:20–14:13). Improper release
 of the drug moiety can have deleterious effects. If, for ex-
 ample, the ADC releases the moiety before reaching the
 target cell, or instead does not release the moiety once in-
 ternalized, then the target cell may not receive the optimal
 dose, and healthy cells may be exposed to the toxic drug.
 J.A. 3106 (209:25–210:17).
    Claim 1, the sole independent claim of the ’039 patent,
 which is representative, reads as follows:
     An antibody-drug conjugate having the formula:
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 4                SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




     or a pharmaceutically acceptable salt thereof,
     wherein:
         Ab is an antibody,
         S is sulfur,
         each —Ww— unit is a tetrapeptide; wherein
         each —W— unit is independently an Amino
         Acid unit having the formula denoted below
         in the square bracket:




         wherein R19 is hydrogen or benzyl,
     Y is a Spacer unit,
     y is 0, 1 or 2,
     D is a drug moiety, and
     p ranges from 1 to about 20,
     wherein the S is a sulfur atom on a cysteine residue
     of the antibody, and
     wherein the drug moiety is intracellularly cleaved
     in a patient from the antibody of the antibody-drug
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.                   5



     conjugate or an intracellular metabolite of the an-
     tibody-drug conjugate.
 ’039 patent col. 331 l. 36–col. 332 l. 40 (emphases added).
     The “Ww” and “Y” limitations of the ’039 patent corre-
 spond to the peptide and spacer units that can be included
 in the linker protein. J.A. 3147–48 (44:22–45:1). The lim-
 itation reciting, “each —Ww— unit is a tetrapeptide,” re-
 quires that the claimed peptide unit is a Gly/Phe-only
 tetrapeptide. Each of the four —W— units must be the
 amino acid depicted in the formula recited in the square
 bracket, wherein “R19 is hydrogen or benzyl.” When R19 is
 hydrogen, the —W— unit is glycine (abbreviated “Gly” or
 “G”), and when R19 is benzyl, the —W— unit is phenylala-
 nine (abbreviated “Phe” or “F”). J.A. 3148 (45:8–21). Ac-
 cordingly, the Ww unit must be a four-amino-acid-long unit
 in which each amino acid is either glycine or phenylala-
 nine—i.e., “a Gly/Phe-only tetrapeptide.” J.A. 3148 (45:8–
 24). Because phenylalanine can exist in either of two dif-
 ferent stereoisomers (spatial arrangements) in addition to
 glycine’s one, there are three different options for each
 amino acid of the claimed Gly/Phe-only tetrapeptide.
 Therefore, the subgenus of peptide units claimed in the
 ’039 patent encompasses 81 (i.e., 34) different species. See
 J.A. 3119 (264:21–24). The “Y is a Spacer unit” limitation
 does not require any specific amino acid sequence, and is
 not at issue in this appeal. ’039 patent col. 331 l. 63.
       The ’039 patent, which was filed in July 2019, is a con-
 tinuing application of U.S. Application 10/983,340, filed in
 November 2004 (“the 2004 application”), from which it
 claims priority. ’039 patent, Related U.S. Application
 Data; see J.A. 5628–5895. The 2004 application describes
 an ADC with the same general formula as the ’039 patent—
 i.e., an antibody, a drug moiety, and a linker protein with
 a peptide unit (“Ww”) that connects the two, J.A. 5711–14,
 but the 2004 application does not explicitly disclose a
 Gly/Phe-only tetrapeptide.      That was the particular
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 6               SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




 difference between the claims of the ’039 patent and the
 2004 application. The 2004 application only discloses that
 the linker protein may contain a peptide unit which is a
 tetrapeptide, and that glycine and phenylalanine are pos-
 sible amino acid options for the peptide unit. J.A. 5722–23.
 The 2004 application does disclose an exemplary tetrapep-
 tide with the formula: glycine, phenylalanine, leucine (ab-
 breviated “Leu” or “L”), glycine (“GFLG”). J.A. 5727; see
 J.A. 5724 (Table IX: “H,” “benzyl,” “isobutyl,” and “H”), J.A.
 3148 (48:8–21 (the amino acid with isobutyl is leucine)),
 but, again, no disclosure of a tetrapeptide containing only
 glycine and phenylalanine. The 2004 application’s descrip-
 tion of specific ADCs is also limited to the class of drugs
 known as dolastatin/auristatin derivatives (“D/A-type
 drugs”). J.A. 5642.
      In November 2011, Daichii invented DS-8201, an ADC
 marketed under the brand name Enhertu®, used to treat
 various types of cancer. J.A. 3113 (240:20–21), 3196
 (239:11–16), 3198 (248:7–8). Enhertu contains an amino
 acid unit with the sequence: Gly-Gly-Phe-Gly, a Gly/Phe-
 only tetrapeptide. J.A. 3196 (240:22–241:7). Enhertu’s
 structure and mechanism of action were made publicly
 known in December 2015. J.A. 3242 (126:8-10). One can-
 not avoid the suspicion that the ’039 patent was filed spe-
 cifically to encompass Enhertu, which of course is
 permissible if it was entitled to a filing date antedating any
 public disclosure of Enhertu. That is the primary issue in
 this case.
                               II
     In October 2020, shortly after the ’039 patent was
 granted, Seagen filed suit against Daichii in the Eastern
 District of Texas, asserting infringement of claims 1–5, 9,
 and 10 of the ’039 patent. J.A. 10846–58. AstraZeneca, a
 later collaborator on Enhertu, subsequently intervened in
 the litigation, joining Daichii as a codefendant. J.A.
 10859–61. In parallel with the district court proceedings,
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.                  7



 in December 2020, Daichii and AstraZeneca petitioned for
 post-grant review (“PGR”) of the asserted claims, which the
 United States Patent Trial and Appeal Board (“the Board”)
 granted.    Daiichi Sankyo, Inc. v. Seagen Inc., No.
 PGR2021-00030 (P.T.A.B. Feb. 14, 2023), Paper 36 (insti-
 tution decision).
     At claim construction before the district court, the par-
 ties disputed the meaning of the term “D is a drug moiety”
 as used in claim 1 of the ’039 patent. J.A. 1286–87. Daichii
 contended that the term should be confined to D/A-type
 drugs, but the district court rejected that argument. J.A.
 at 1286. The district court reasoned that because neither
 the text of the claim nor the specification requires the drug
 used in the ADC to be a particular type of drug, “D is a drug
 moiety” should be given its plain meaning, encompassing
 any drug moiety. J.A. at 1287–92.
      At trial, the parties disputed both validity and infringe-
 ment. The issues, in part, 1 converged on whether the 2004
 application provided written description support for a
 Gly/Phe-only tetrapeptide, the Ww peptide unit recited in
 claim 1, thus entitling the ’039 patent to the 2004 applica-
 tion’s November 2004 priority date. Because the jury had
 found that Enhertu meets each limitation of one of the as-
 serted claims, the priority date of the ’039 patent became
 critical: If the 2004 application provided adequate written
 description support for its claims, Enhertu would infringe
 a valid ’039 patent. But if the 2004 application did not pro-
 vide adequate written description support for its claims,
 the public disclosure of Enhertu would anticipate them,
 and thus invalidate, with, accordingly, no infringement.




     1  Daichii presented separate arguments as to why
 Enhertu did not infringe the ’039 patent, but those argu-
 ments are not at issue in this appeal.
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 8                      SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




     The below timeline illustrates the key events and
 dates.
     2004 Application     Enhertu Publicly     039 Application   Seagen Files Suit
          Filed              Disclosed              Filed         Against Daichii



        Nov. 2004            Dec. 2015            July 2019         Oct. 2020
     At the conclusion of a five-day trial, the jury reached a
 verdict in Seagen’s favor, finding that (1) Daichii failed to
 prove any of the asserted claims were invalid, (2) Seagen
 proved Daichii’s Enhertu infringed at least one of the as-
 serted claims and that such infringement was willful, and
 (3) Seagen proved Daichii owed an upfront royalty of
 $41,820,000 (based on Daichii’s net revenue during the pe-
 riod of infringement) and an 8% running royalty. J.A. 53–
 60. Daichii moved for JMOL, arguing, in relevant part,
 that (1) the claims were not supported by a sufficient writ-
 ten description, (2) the claims were not enabled, and (3)
 there was not a logical nexus between the licenses relied
 upon by Seagen’s damages expert and the hypothetical ne-
 gotiation between Seagen and Daichii in arriving at the 8%
 royalty rate. J.A. 2134–65, 2191–96. The district court de-
 nied Daichii’s motion for JMOL, J.A. 32–49, and entered
 final judgment. J.A. 50–52.
    Daichii timely appealed. We have jurisdiction under
 

28 U.S.C. § 1295

(a)(1).
     In January 2024, in the separate PGR proceeding, the
 Board issued a final written decision holding the claims as-
 serted in the district court litigation unpatentable on the
 same grounds asserted by Daichii in the district court case.
 Daiichi, No. PGR2021-00030, Paper 57. Seagen has ap-
 pealed the Board’s decision to this court, which has been
 designated as a companion case to the instant appeal, and
 which we dismiss as moot in a separate decision. See Sea-
 gen Inc. v Daichii Sankyo, Inc. No. 24-1878, slip. op. at 3
 (Fed. Cir. 2025).
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.                  9



                          DISCUSSION
     On appeal, Daichii argues that the district court erred
 in denying JMOL on each of the above issues that Daichii
 raised in its post-trial briefing; it does not challenge the
 jury’s finding that Enhertu infringes at least one claim of
 the ’039 patent. Daichii Br. 38–76. Daichii’s liability there-
 fore rises and falls with its invalidity arguments. Because
 the record does not contain substantial evidence that the
 2004 application provides written description support for a
 Gly/Phe-only tetrapeptide or that the ’039 patent enables a
 skilled artisan to make and use the full scope of the claimed
 ADCs without undue experimentation, we conclude that all
 of its claims are invalid. We therefore need not reach
 Daichii’s challenge to Seagen’s damages award.
      We review the denial of a motion for JMOL under re-
 gional circuit law. Freshub, Inc. v. Amazon.com, Inc., 

93 F.4th 1244, 1249

 (Fed. Cir. 2024). JMOL is appropriate
 only where “the facts and inferences point so strongly and
 overwhelmingly in favor of one party that the court con-
 cludes that reasonable jurors could not arrive at a contrary
 verdict.” Orion IP, LLC v. Hyundai Motor Am., 

605 F.3d 967, 973

 (Fed. Cir. 2010) (applying Fifth Circuit law). We
 affirm a district court’s denial of judgment as a matter of
 law when there was substantial evidence to support the
 jury’s verdict. Power-One, Inc. v. Artesyn Techs., Inc., 

599 F.3d 1343

, 1350–51 (Fed. Cir. 2010) (applying Fifth Circuit
 law). Federal Circuit law applies to issues of substantive
 patent law. See e.g., Colibri Heart Valve LLC v. Medtronic
 CoreValve, LLC, 

143 F.4th 1367

, 1376 (Fed. Cir. 2025).
                    I. Written Description
     In order for a later-filed patent “to gain the benefit” of
 an earlier application’s priority date, inter alia, the earlier
 application must disclose an invention “in the manner pro-
 vided by section 112(a) [of the Patent Act].” Lockwood v.
 Am. Airlines, Inc., 

107 F.3d 1565, 1571

 (Fed. Cir. 1997); 

35 U.S.C. § 120

. Section 112(a) of the Patent Act provides that
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 10               SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




 a patent “specification shall contain a written description
 of the invention.” 

35 U.S.C. § 112

(a). A specification ade-
 quately describes an invention when it “reasonably conveys
 to those skilled in the art that the inventor had possession
 of the claimed subject matter as of the filing date.” Ariad
 Pharms., Inc. v. Eli Lilly & Co., 

598 F.3d 1336, 1351

 (Fed.
 Cir. 2010) (en banc) (citation omitted). “[T]he hallmark of
 written description is disclosure” in the “four corners of the
 specification.” 

Id.

 Whether a patent complies with the
 written description requirement is a question of fact, and
 “we review a jury’s determinations of facts relating to com-
 pliance with the written description requirement for sub-
 stantial evidence.” Juno Therapeutics, Inc. v. Kite Pharma,
 Inc., 

10 F.4th 1330

, 1335–36 (Fed. Cir. 2021) (quoting Ar-
 iad, 

598 F.3d at 1355

).
     Daichii argues that no reasonable jury could have
 found that the 2004 application sufficiently discloses the
 claimed 81-member subgenus 2 of Gly/Phe-only tetrapep-
 tides corresponding to the “—Ww— unit” recited in claim 1.
 We agree and therefore conclude that substantial evidence
 does not support the jury’s finding of adequate written de-
 scription.
                                A
     The 2004 application only broadly outlines the possible
 structures of the Ww peptide unit that are included in the
 linker protein. It states that “when present,” the peptide
 unit “Ww” can be any of “a dipeptide, tripeptide, tetrapep-
 tide, pentapeptide, hexapeptide, heptapeptide, octapep-
 tide, nonapeptide, decapeptide, undecapeptide or



      2  We use the term “subgenus” here because the par-
 ties have. Claim 1 is of course a subgenus of a larger genus
 described in the 2004 application. But claim 1 might
 equally be called a genus, as it certainly is, in fact, a rather
 large one, as can be seen here.
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.                11



 dodecapeptide unit.” J.A. 5722–23. That is, if the peptide
 unit is included in the ADC described in the 2004 applica-
 tion, it can include anywhere from two to twelve amino ac-
 ids. J.A. 5722–23. The 2004 application then describes 39
 different amino acids that can constitute the R19 group,
 which determines the identity of the —W— unit. J.A.
 5723. When different isomers of the identified amino acids
 are included, there are 83 unique options for each “—W—”
 unit. See J.A. 3243 (130:16–132:24). Accordingly, just with
 respect to tetrapeptides, over 47 million different tetrapep-
 tide units (i.e., 834) are encompassed in the 2004 applica-
 tion. 3 Seagen and Daichii’s witnesses agreed on this point
 at trial. J.A. 3120 (265:3–10) (lead named inventor Dr. Pe-
 ter Senter admitting that “[t]he number of options” in the
 2004 application “for just the tetrapeptide category alone
 is over 47 million”), 3243 (132:19–24) (Daichii expert Dr.
 John Lambert stating that in “[I]n tetrapeptides alone[,]
 there are 47 million options” possible in the 2004 applica-
 tion).
      But such evidence is insufficient to support the jury’s
 finding of adequate written description in the 2004 appli-
 cation. As we have consistently held, a patent’s disclosure
 of a “broad genus,” without more, is inadequate to satisfy
 the written description requirement for claims directed to
 a “particular subgenus or species contained therein.” See,
 e.g., Novozymes A/S v. DuPont Nutrition Biosciences APS,
 

723 F.3d 1336, 1346

 (Fed. Cir. 2013).
     Our predecessor court’s decision in In re Ruschig is in-
 structive. 

379 F.2d 990

 (CCPA 1967). There, the claim re-
 cited a single compound, which was “encompassed” in the
 application’s “general disclosure of . . . something like half


     3   The number of peptides encompassed by the 2004
 application with each additional amino acid unit grows ex-
 ponentially—e.g., almost four billion pentapeptides are in-
 cluded (83^5).
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 12              SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




 a million possible compounds.” 

Id. at 993, 996

. The appli-
 cant urged that such a disclosure was adequate. The Court
 of Customs and Patent Appeals (“CCPA”) rejected that ar-
 gument, explaining that claims to a particular species or
 subgenus require “reasonably specific supporting disclo-
 sure” to show that the inventor possessed the specific com-
 pound. 

Id. at 994

.
      Similarly, here, it was undisputed at trial that the 2004
 application broadly discloses an extraordinary number of
 peptide units—e.g., with respect to just the tetrapeptides
 covered, over 47 million species based on 83 different
 amino acids. J.A. 3120 (265:3–10), 3243 (132:19–24). And
 although the 81 claimed Gly/Phe-tetrapeptides are “encom-
 passed” within those 47 million, see J.A. 3148 (46:8–11),
 3256–57 (184:23–185:22), they are merely an infinitesimal
 fraction of those peptide units generally included. Under
 Ruschig, such evidence does not provide “reasonably spe-
 cific supporting disclosure” for the claimed subgenus of 81
 Gly/Phe-only tetrapeptides. 379 F.2d at 993–94. Accord-
 ingly, the above evidence was not substantial evidentiary
 support for the jury’s finding of adequate written descrip-
 tion.
      The ’039 patent thus cannot claim priority from the
 2004 application, meaning that the December 2015 public
 disclosure of Enhertu, which was found to contain each lim-
 itation of at least claim 1 (the sole independent claim), an-
 ticipates, and thus invalidates the ’039 patent. Bristol-
 Myers Squibb Co. v. Ben Venue Laby’s, Inc., 

246 F.3d 1368

,
 1378 (Fed. Cir. 2001 (“[T]hat which would literally infringe
 if later anticipates if earlier.”).
      Attempting to distinguish cases like Ruschig, Seagen
 principally relies on another of our predecessor court’s de-
 cisions, In re Driscoll, 

562 F.2d 1245

 (CCPA 1977). Seagen
 Br. 35–36. There, the patent claim recited a subgenus of
 chemical compounds with a specific moiety, and an earlier
 application which the patent claimed priority from recited
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.                 13



 fourteen moieties, including the one claimed. Driscoll, 

562 F.2d at 1249

. The CCPA held that the earlier application
 provided written description support for the claim because
 “the exact subgenus claimed is clearly discernible in the
 generalized formula” and therefore “a skilled artisan would
 recognize from the disclosure of [the specification]” that the
 inventor possessed each of the “fourteen distinct [moie-
 ties].” 

Id.

 at 1249–50.
     Seagen’s reliance on Driscoll is misplaced. Here, as
 discussed, there are effectively countless peptide units en-
 compassed by the 2004 application—a far cry from the 14
 moieties individually listed in the specification in Driscoll.
 And the claimed subgenus of 81 Gly/Phe-only tetrapeptides
 is never mentioned in the 2004 application, and therefore
 is not “clearly discernible in the generalized formula” in the
 way the CCPA found sufficient in Driscoll. 

Id. at 1250

.
     The conclusion that the 2004 application does not pro-
 vide adequate written description support is further sup-
 ported by admissions from the ’039 patent’s named
 inventors that they had never contemplated an ADC with
 a Gly/Phe-only tetrapeptide as of the 2004 application’s pri-
 ority date. See Nuvo Pharms. (Ireland) Designated Activity
 Co. v. Dr. Reddy’s Lab’ys Inc., 

923 F.3d 1368, 1381

 (Fed.
 Cir. 2019) (inventor testimony can “illuminate[] the ab-
 sence of critical [written] description”).
      Specifically, lead inventor Dr. Senter admitted that the
 “first time . . . [he] ever saw a G/F-only tetrapeptide in an
 ADC . . . was in Dacihii Sankyo’s Enhertu,” which was pub-
 licly disclosed in 2015. J.A. 3119 (261:22–23). Similarly,
 inventor Dr. Svetlana Doronina admitted that “the claims
 of the ’039 patent,” filed in 2019, were “the first time” she
 had “seen described a genus of tetrapeptide linkers that’s
 limited to glycine [and] phenylalanine.” J.A. 3295 (74:9–
 12). Named inventors Dr. Brian Toki and Dr. Toni Kline
 testified to similar effect. J.A. 3292 (62:13–63:4) (Dr. Toki);
 3297 (84:1–9). Because “one cannot describe what one has
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 14               SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




 not conceived,” Fiers v. Revel, 

984 F.2d 1164, 1171

 (Fed.
 Cir. 1993), the inventors’ admissions bolster the conclusion
 that they did not possess the claimed subgenus of Gly/Phe-
 only tetrapeptides as of the November 2004 priority date.
     Dr. Senter’s additional testimony that the inventors
 created ADC linkers comprising “almost any peptide” prior
 to the 2004 application’s priority date is not sufficient to
 overcome his above admission for the same reason that the
 disclosures in the 2004 application are inadequate. Seagen
 Br. 38 n.12; J.A. 3110 (229:6–8). Dr. Senter’s testimony
 does not show that the 2004 application provides “reason-
 ably specific supporting disclosure” for the claimed
 Gly/Phe-only tetrapeptides, or that the inventors had pos-
 session of that subgenus of tetrapeptides. Ruschig, 

379 F.2d at 994

.
                               B
     Separately, Seagen asserts that the finding of adequate
 written description was supported by substantial evidence
 because the jury heard testimony that the 2004 application
 would “direct[]” a skilled artisan to the claimed subgenus
 of Gly/Phe-only tetrapeptides. Seagen Br. 40–45. We have
 often analyzed such theories by analogizing a genus and its
 constituent subgenuses or species to a forest and trees:
      It is an old custom in the woods to mark trails by
      making blaze marks on the trees. It is no help in
      finding a trail or in finding one’s way through the
      woods where the trails have . . . not yet been made
      . . . to be confronted simply by a large number of
      unmarked trees. . . . We are looking for blaze marks
      which single out particular trees.
 Ruschig, 379 F.2d at 994–95 (emphasis added).
    Seagen presented its “blaze-mark” theory through Dr.
 Carolyn Bertozzi. Dr. Bertozzi provided considerable tes-
 timony purporting to show how a skilled artisan could have
 made certain changes in the compounds disclosed to arrive
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.                  15



 at a compound within the scope of the claimed subgenus.
 Specifically, Dr. Bertozzi testified that, based on the 2004
 application’s disclosure of the tetrapeptide, GFLG, to-
 gether with several tripeptides where phenylalanine is at
 the second position, a skilled artisan would understand
 that “F at [position] 2” was a possible substitution for leu-
 cine in GFLG. J.A. 3299–3300 (91:19–93:5). The highlight
 of that testimony stated that it would be a “straightforward
 leap” for a skilled artisan “to go from GFLG . . . to a
 [tetra]peptide that’s all G and F.” J.A. 3149 (50:25–51:2).
      But that testimony dooms Seagen’s case. That which
 one must leap to is obviously not there. It is thus axiomatic
 that a skilled artisan need not make a “leap” to “a
 [tetra]peptide that’s all G and F” if the skilled artisan were
 to understand that the 2004 application itself disclosed
 such a peptide. J.A. 3149 (50:25–51:2). That testimony is
 accordingly self-defeating: It in effect admits that a skilled
 artisan would not understand the inventors of the 2004 ap-
 plication to have “possession” of GFFG. Ariad, 

598 F.3d at 1351

; see also Lockwood, 

107 F.3d at 1572

. (“It is not suf-
 ficient for purposes of . . . written description . . . that the
 disclosure, when combined with the knowledge in the art,
 would lead one to speculate as to modifications that the in-
 ventor might have envisioned, but failed to disclose.”); Re-
 gents of the Univ. of Minn. v. Gilead Scis. Inc., 

61 F.4th 1350, 1358

 (Fed. Cir. 2023) (lack of written description
 where the priority documents blaze a trail “that runs close
 by the later-claimed tree,” but “‘do[ ] not direct one to the
 proposed tree in particular, and do[ ] not teach the point at
 which one should leave the trail to find it.’” (alterations in
 original) (quoting Fujikawa v. Wattanasin, 

93 F.3d 1559, 1571

 (Fed. Cir. 1996))).
     In sum, the 2004 application’s broad disclosure of effec-
 tively countless options for the peptide unit fails to show
 the subgenus of Gly/Phe-only tetrapeptides claimed in the
 ’039 patent. Moreover, there are no adequate blaze marks
 that would lead a skilled artisan to the 81-member
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 16              SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




 Gly/Phe-only tetrapeptide subgenus or any species within
 it. A reasonable jury therefore did not have sufficient evi-
 dence to find that the ’039 patent provided written descrip-
 tion support for what it claimed. Accordingly, the district
 court erred in denying Daichii’s motion for JMOL on that
 ground.
                       II. Enablement
      Another issue at trial relating to validity was whether
 the ’039 patent satisfied the enablement requirement. Spe-
 cifically, the parties disputed whether the ’039 patent suf-
 ficiently enabled a skilled artisan to make and use ADCs
 with the claimed structure and function. Seagen’s wit-
 nesses asserted that undue experimentation was not nec-
 essary to make and use the full scope of claimed ADCs.
      The enablement requirement is distinct from the writ-
 ten description requirement. Ariad, 

598 F.3d at 1351

. A
 patent’s specification must describe the invention and “the
 manner and process of making and using it, in such full,
 clear, concise, and exact terms as to enable any person
 skilled in the art to which it pertains . . . to make and use
 the same.” 

35 U.S.C. § 112

(a). This means that “the spec-
 ification must enable the full scope of the invention as de-
 fined by its claims,” allowing for “a reasonable amount of
 experimentation.” Amgen v. Sanofi, 

598 U.S. 594

, 610, 612
 (2023). Put differently, “[t]o be enabling, the specification
 of a patent must teach those skilled in the art how to make
 and use the full scope of the claimed invention without un-
 due experimentation.” MagSil Corp. v. Hitachi Glob. Stor-
 age Techs., Inc., 

687 F.3d 1377, 1380

 (Fed. Cir. 2012)
 (cleaned up). “Whether a claim satisfies § 112’s enable-
 ment requirement is a question of law [that] we review de
 novo; however, in the context of a jury trial, we review the
 factual underpinnings of enablement for substantial evi-
 dence.” Trs. of Bos. Univ. v. Everlight Elecs. Co., 

896 F.3d 1357, 1361

 (Fed. Cir. 2018) (citation omitted).
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.               17



     Daichii argues that the ’039 patent failed to provide an
 enabling disclosure of the subject matter of the claims. We
 agree with Daichii. No reasonable jury could have found
 that the specification of the ’039 patent teaches a skilled
 artisan how to make and use the full scope of the claimed
 invention. We therefore conclude that substantial evidence
 does not support the jury’s finding of enablement for any of
 the asserted claims.
     The Supreme Court’s decision in Amgen v. Sanofi,
 unanimously affirming our decision, is relevant here. In
 Amgen, the patents claimed “the entire genus of antibod-
 ies” that (1) bind to specific amino acid residues on a pro-
 tein and (2) block that protein from binding to receptors
 involved in removing a certain harmful cholesterol from
 the bloodstream. 598 U.S. at 602 (internal quotations
 omitted). It was undisputed that the claims covered “at
 least millions” of antibodies with the recited functionali-
 ties. Id. at 613. The patents’ specification identified 26
 antibodies that perform the two recited functions along
 with their relevant structures, id. at 602–03, as well as two
 methods for testing whether a particular antibody per-
 forms the two recited functions. Id. at 603.
      Amgen, the patent owner, contended that the specifi-
 cation’s disclosure of the two methods was sufficient to en-
 able a skilled artisan to make and use the full scope of the
 claimed invention. Id. at 613–14. The Court rejected that
 argument. It explained that—in light of the unpredictabil-
 ity of antibody science where even a minor change of chem-
 ical structure can alter an antibody’s functionality—
 without disclosing “a quality common to every functional
 embodiment,” both methods effectively amounted to telling
 a skilled artisan to perform “trial-and-error discovery” to
 determine if any one antibody may possess the recited
 functionalities. Id. at 600, 614–15.
     So too here. The district court construed the limitation
 “D is a drug moiety” to encompass any type of drug moiety.
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 18              SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.




 J.A. 1287–92. Further, the ’039 patent recites a functional
 limitation: “the drug moiety is intracellularly cleaved in a
 patient from the antibody.” ’039 patent col. 332 ll. 37–39;
 see Nevro Corp. v. Bos. Sci. Corp., 

955 F.3d 35, 39

 (Fed. Cir.
 2020) (“[A] functional term [is] defined by what it does ra-
 ther than what it is.”). Taken together, these two limita-
 tions mean that the ’039 patent covers an ADC containing
 any drug moiety with the recited function of cleaving in a
 patient. But the specification does not disclose “a quality
 common to every functional embodiment” of the patent.
 Amgen, 598 U.S. at 614. Rather, it is undisputed that ADC
 science was so unpredictable that a skilled artisan would
 be required to use an assay to test whether any given ADC
 with a given drug moiety meets that functional limitation.
 J.A. 3296 (78:21–79:1) (inventor Dr. Kline admitting that a
 skilled artisan would need to conduct an assay to deter-
 mine whether a given ADC is “is going to be successfully
 intracellularly cleaved”); Seagen Br. 56–57 (listing various
 assays a skilled artisan would know to use to determine
 whether a given ADC’s drug moiety is intracellularly
 cleaved). Therefore, just as in Amgen, a skilled artisan
 would be left to perform far-ranging “trial-and-error discov-
 ery” to make and use the ADCs within the scope of the ’039
 patent. Amgen, 598 U.S. at 615. 4 Under these circum-
 stances, where the scale of trial and error is so vast and the
 science so unpredictable, such experimentation is undue
 under Amgen, see id. Thus, the jury did not have substan-
 tial evidence to conclude that the ’039 patent was enabled.




      4 The amount of trial-and-error discovery that would
 be required to determine whether an ADC with a given
 drug intracellularly cleaves is further increased by the
 need to test each ADC with the recited structural limita-
 tions.
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 SEAGEN INC. v. DAIICHI SANKYO COMPANY, LTD.                19



                         CONCLUSION
      We have considered Seagen’s remaining arguments
 and find them unpersuasive. For the foregoing reasons, we
 reverse the district court’s denial of judgment as a matter
 of law that the ’039 patent is not invalid for failure to meet
 the written description and enablement requirements. The
 ’039 patent is therefore invalid. We vacate the jury’s find-
 ing of willful infringement and assessment of damages.
                        REVERSED