The Federal Circuit issued a precedential opinion in In re: Xencor, Inc.[1] concerning written support for Jepson claims. The decision affirms the decision of the Appeals Review Panel (ARP) of the USPTO, which held that the pending claims at issue in U.S. Patent Application
No. 16/803,690 are unpatentable for lacking written description. Although there were two different claims on appeal in In re: Xencor (a “Jepson claim” and a “method claim”), this blog post will focus only on the Jepson claim.
1. Refresher: What is a Jepson Claim?
A Jepson claim is a type of patent claim that describes an improvement over the prior art.
In sum, a Jepson claim has a certain structure: (i) a preamble that describes known features of the subject matter; (ii) a transitional phrase such as “wherein the improvement comprises”; and (iii) a recitation of the new or improved features of the subject matter.
2. The Jepson Claim at Issue in Xencor
The Jepson claim at issue in Xencor, claim 8, was directed to a method of treating a patient by administering a modified antibody. The method had the purported beneficial effect of enhancing serum half-lives while reducing treatment frequency. The claim is recited below:
8. In a method of treating a patient by administering an anti-C5 antibody with an Fc domain, the improvement comprising said Fc domain comprising amino acid substitutions M428L/N434S as compared to a human Fc polypeptide, wherein numbering is according to the EU index of Kabat, wherein said anti-C5 antibody with said amino acid substitutions has increased in vivo half-life as compared to said antibody without said substitutions.
3. The Preamble of a Jepson Claim Requires Written Description
The issue on appeal regarding Xencor’s claim 8 was whether the preamble—specifically, the reference to an anti-C5 antibody—needed written description, and if so, whether Xencor satisfied this requirement.
Xencor argued that the specification need only provide written support for the improvement recited in the Jepson claim, and hence not the preamble itself. The ARP disagreed, and argued that Jepson claim preambles are part of the invention and therefore require written description.
After analyzing the requirements of written description under 35 U.S.C. 112 and assessing relevant case law, the Federal Circuit agreed with the ARP that a Jepson claim preamble requires written description. As the Federal Circuit reasoned, the preamble in a Jepson claim defines the claimed invention and therefore limits the claim scope. The court continued: “[t]he invention is not only the claimed improvement, but the claimed improvement as applied to the prior art, so the inventor must provide written description sufficient to show possession of the claimed improvement to what was known in the prior art.”[2]
The Federal Circuit analogized Xencor’s Jepson claim to a Jepson claim directed to a time machine:
A patentee cannot be permitted to use a Jepson claim to avoid the requirement that she be in possession of the claimed invention simply by asserting something is well-known in the art. For example, a patentee cannot obtain a Jepson claim with a preamble that says that a time machine is well-known in the art without describing a time machine, in sufficient detail to make clear to a person of ordinary skill in the art that the inventor is in possession of such a time machine.[3]
4. The Jepson Claim in Xencor Did Not Satisfy the Written Description Requirement
As the Federal Circuit noted, in order to provide sufficient written description for a Jepson claim, the applicant must establish that what is well-known in the prior art is indeed well-known. Both the Federal Circuit and the PTAB agreed that Xencor failed to demonstrate that the anti-C5 antibody limitation recited in the Jepson preamble was in fact well-known. In making this determination, the Board examined, among other things, the few examples of anti-C5 antibodies in the prior art and the large number of possible antibodies in the genus.
5. Takeaways
The Federal Circuit’s holding requires that Jepson claims have full written support to demonstrate that the inventor had possession of the invention as applied to the prior art. As a result, applicants should consider sufficiently describing the prior art recited in the preamble of a Jepson claim or demonstrate that the prior art is “well-known” to a person of ordinary skill in the art. As the court recognized, the amount and content of the disclosure necessary to confer adequate written description will vary, depending on several factors (e.g., how new the technology is, the unpredictability of the art, and the level of knowledge of the person of ordinary skill in the art). For example, for a Jepson claim directed to an improved front-loading washing machine, simply using the term, “front-loading washing machine,” would have likely been insufficient in the 1800s without more explanation. Today, however, the term, “front-loading washing machine,” is likely well-known and probably needs no further explanation.
[1] In re Xencor Inc., No. 2024-1870 (Fed. Cir. Mar. 13, 2025).
[2] Id. at 17 (emphasis in original).
[3] Id. at 17-18.
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