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The Federal Circuit recently issued a precedential decision, Seagen Inc. v. Daiichi Sankyo Co., (Fed. Cir. December 2, 2025), that tightens the standard for demonstrating written description support for sub-genus or species claims based on an earlier broad genus disclosure.
Seagen's 2004 application disclosed more than 47 million possible tetrapeptide linker sequences for antibody-drug conjugates (ADCs). In 2020, Seagen obtained a patent from a continuation application of the 2004 application. The 2020 patent claimed a much narrower sub-genus—tetrapeptides containing only glycine and phenylalanine (81 possible sequences). The Federal Circuit suspected that the patent was aimed at covering Daiichi's cancer drug, Enhertu®, which was publicly disclosed in 2015.
Following a jury trial in the Eastern District of Texas (EDTX), the district court denied Daiichi's JMOL that the claims of the 2020 patent were invalid for lack of written description. The Federal Circuit found that the district court erred in denying Daiichi's motion for JMOL, stating that no reasonable jury could conclude that the 2004 application (47 million possible tetrapeptide linker sequences for ADCs) demonstrated possession of the later-claimed sub-genus (81 possible sequences). The Court emphasized that the disclosure of "effectively countless options" failed to show the 81-member subgenus claimed in the 2020 patent.
The Federal Circuit rejected Seagen's "blaze mark" theory, which relied on a disclosed exemplary tetrapeptide (GFLG) and expert testimony characterizing a switch from GFLG to a sub-genus tetrapeptide (GFFG) as a "straightforward leap." Emphasizing the need for blaze marking in the original disclosure, the Federal Circuit indicated that the landing point of the straightforward leap was not in the original disclosure The Federal Circuit also found that Seagen's expert's testimony "doomed" its case, effectively admitting that GFFG was not in the original application, given that "a skilled artisan need not make a 'leap' to 'a [tetra]peptide that's all G and F'" if it had been in the original disclosure.
Evidence regarding what an inventor might have envisioned or what a skilled artisan might have found obvious based on the original 2004 application cannot substitute for lack of written description in the application itself.
The Federal Circuit's decision underscores that applicants cannot rely on expert testimony to supply missing disclosure. To preserve flexibility for future claim strategies, original applications must contain sufficient "blaze marks" pointing to the later-claimed sub-genus or species.
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