Written description and genus claims: lessons from In re: BAC IP B.V.

On August 5, 2025, in In re: BAC IP B.V., No. 24-2124, the Court of Appeals for the Federal Circuit (CAFC) affirmed the rejection of broad genus claims for lack of written description under 35 U.S.C. § 112, first paragraph. The opinion underscores that satisfying the written description requirement depends on what the specification affirmatively demonstrates the applicant possessed as of the filing date.

The claims and the rejection

In re: BAC IP B.V. concerns US Patent Application No. 16/282,082, directed to methods for purifying human IgG antibodies using variable heavy-heavy (VHH) antigen-binding proteins. Independent claim 77, representative of the rejected claims, recites a method involving binding human IgG to a VHH antigen-binding protein comprising four framework regions (FR1-FR4) and three complementarity determining regions (CDR1-CDR3), linked in a particular order. The CDR3 amino acid sequence is defined as Arg-Arg-Phe-Gly-Ser-Ser-Glu-Trp-Asp-Tyr (SEQ ID NO:108). The claim also includes a functional limitation: the VHH protein must bind human IgG but not IgG from certain non-human species (e.g., mouse, rat, goat, etc.).

The claims were rejected for lack of written description. While the claims covered a broad genus of VHH antigen-binding proteins, the specification disclosed a single species meeting all of the claimed requirements and failed to identify any unifying structural features across the genus. On appeal, the Patent Trial and Appeal Board (PTAB) affirmed, finding that the full scope of the claimed genus was not adequately supported.

The Federal Circuit’s reasoning

BAC challenged the PTAB’s conclusion that the claims were overly broad, arguing that (1) the specification disclosed only 15 VHH antigen-binding proteins with the claimed binding specificity, suggesting a narrow genus; and (2) the specification explained how to identify additional proteins with the required characteristics.

The Federal Circuit rejected both arguments. It first emphasized that the written description requirement turns on whether the inventor had possession of the invention, which is defined by the claims. Because claim 77 was not limited to the 15 disclosed proteins and only one of them satisfied all of the claim’s structural and functional requirements, the Court found the disclosure insufficient.

The Court reaffirmed that a genus claim must be supported by either (1) a representative number of species, or (2) common structural features that enable a skilled artisan to “visualize or recognize” the scope of the genus (Ariad Pharms., Inc. v. Eli Lilly & Co., 598 F.3d 1336, 1350 (Fed. Cir. 2010) (en banc), quoting Regents of the Univ. of California v. Eli Lilly & Co., 119 F.3d 1559 (Fed. Cir. 1997)). In this case, only the CDR3 region was structurally defined in the claim; CDR1 and CDR2 were unspecified. Although the application disclosed 49 possible amino acid sequences for each, it neither stated that the sequences constituted the entirety of the CDR1 nor CDR2 regions nor demonstrated that the variants would satisfy the claimed functional criteria.

The Court further held that conveying how to identify additional proteins does not satisfy § 112 where a broad claim is supported by only one fully described species. Accordingly, the Court affirmed the PTAB’s conclusion that the claimed genus lacked sufficient written description.

Practical takeaways

This decision reinforces a fundamental principle for practitioners: when drafting genus claims, the specification must demonstrate possession of the claimed invention. A single example will seldom satisfy § 112 unless it either reflects the full scope of the genus or clearly articulates the structural features that unify its members. Functional limitations (e.g., binding to human IgG) cannot substitute for defining the genus structurally. Moreover, describing a method for identifying additional variants does not cure a lack of written description. Applicants must establish that they possessed the claimed subject matter at the time of filing, not merely the tools to uncover it.

Conclusion

While In re: BAC IP B.V. is nonprecedential, its message is consistent with long-standing Federal Circuit doctrine: broad genus claims require concrete structural disclosure commensurate with their scope. As patent prosecutors continue to navigate the balance between broad protection and compliance with § 112, this decision reinforces that functional intent must be anchored by clearly disclosed species or shared structural features.

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