So-called "Jepson" claims have long been considered to be written in a high-risk claim format in U.S. patent practice. While they are more common in other jurisdictions and even encouraged in certain USPTO guidance, U.S. practitioners have traditionally avoided them due to their restrictive nature. A Jepson claim structure designates the preamble as admitted prior art, leaving only the limitations following "wherein the improvement comprises" to define the claimed invention. In In re Xencor,1 the U.S. Court of Appeals for Federal Circuit has amplified these risks by holding that the preamble of a Jepson claim must be supported by the same sufficient written description required under 35 U.S.C. § 112 for the invention itself. This decision significantly impacts patent applicants, as it effectively mandates disclosure of the inventive improvement as well as the admitted prior art content recited in the claim preamble.
Xencor was an appeal concerning a patent application relating to methods of treating patients with an anti-C5 antibody having certain amino acid substitutions in the Fc domain. In typical Jepson claim format, the claim preamble recited the known approach of treating a patient by administering an anti-C5 antibody, and the improvement portion of the claim recited the specific substitution that allegedly increased the antibody's in vivo half-life. Xencor argued that the relevant portion of the claim requiring 35 U.S.C. § 112-compliant written description support was only the improvement since the invention in a Jepson claim is limited to that improvement. The USPTO examiner and the Patent Trial and Appeal Board (PTAB or Board) disagreed, so Xencor appealed the Board's decision to the Federal Circuit.
While Xencor's appeal to the Federal Circuit was pending, the Patent Office asked the court to remand the case back to the PTAB so that its Appeals Review Panel (ARP) could consider the issues raised by Xencor in the first instance. The ARP analyzed whether the Jepson claim preamble was substantively limiting and, after finding that it was limiting, determined that Xencor's application did not support the preamble with sufficient written description as required by Section 112 of the Patent Statute.
The Federal Circuit agreed with the PTAB's ARP decision. The court ruled that the entirety of the Jepson claim, including the preamble recitation of a known anti-C5 antibody, constituted the claimed invention for written description purposes. The Federal Circuit observed that the specification did not list or describe a representative number of species for the claimed anti-C5 antibodies, and thus the entire genus was not properly supported. According to the opinion, even if a claim's Jepson preamble describes admitted prior art, the applicant cannot simply rely on a general statement that such subject matter is already well known. Instead, if the claim covers the prior art as applied to the improvement, then the specification must demonstrate the applicant's possession of that entire scope, including any recited features in the preamble.
Making the ruling even more consequential, the Federal Circuit highlighted a hypothetical involving a time travel machine. Specifically, if someone tried to recite such a time machine in a Jepson preamble without describing or possessing such a device, they could theoretically obtain a patent on something the inventor does not actually possess. The court concluded that requiring written description support for every aspect of the Jepson claim, including the preamble, is a necessary safeguard against applicants patenting fictional or overly broad subject matter that they never actually invented.
Currently, most U.S. practitioners avoid the use of Jepson format because it necessarily concedes that everything recited in the preamble is prior art. As such, Jepson claims limit the scope of protection to whatever improvement follows the "wherein the improvement comprises" clause. However, there are certain scenarios, particularly mechanical or method improvements, where a Jepson claim can streamline prosecution. Additionally, USPTO Rule 37 CFR § 1.75(e) explicitly encourages Jepson claims where an invention is an improvement. However, while most inventions can be regarded as being "improvements" the use of the Jepson claim format remains rare.
Jepson claims have been considered as limiting, but prior to the Xencor decision they were not generally believed to require written description for the entire preamble's subject matter. The logic for this understanding was that everything in the preamble is admitted as being prior art, so the applicant need only satisfy 35 U.S.C. § 112 written description requirement for the new or improved features. That view is now definitely incorrect. The Federal Circuit's reasoning effectively states that an applicant must demonstrate, in their specification, that the preamble subject matter is described in sufficient detail to establish that the entire scope of the claimed invention is "possessed" by the inventor as of the filing date.
There may be existing issued patents containing Jepson claims that do not fully disclose each element recited in the preamble. Xencor invites questions about the validity of those granted Jepson claims if challenged. Additionally, we can assume that, going forward, lack of written description support for a Jepson claim's preamble will be raised in patent validity challenges both before the PTAB and in district court litigation.
Going forward, requiring written description support for the subject matter disclosed in the preamble makes any use of Jepson claims even more problematic. If a patent drafter chooses to use the Jepson claim format, then they must thoroughly describe the admitted prior art portion, including representative embodiments in the specification. Alternatively, they may demonstrate (possibly during prosecution) that the recited preamble subject matter was widely known in the field by providing robust evidence of that well-known status. The Federal Circuit used the term "automobile" to demonstrate something that is so well known at this time that it need not require further elaboration in the specification.
In addition, an applicant has to accept the usual consequence that everything in the preamble is an express admission of prior art. These issues together make the use of Jepson claims even more unattractive than it had already been. Any gap in the specification's disclosure of the admitted prior art might render the entire claim unpatentable or invalid under the Section 112 written description requirement. Most U.S. patent practitioners will likely refrain from any use of Jepson claims except in an exceedingly narrow set of circumstances.
Traditionally, U.S. practitioners have reserved Jepson claims for mechanical improvements and occasionally for method improvements, often to accelerate prosecution or meet the examiner's preference. Now, the safer option is to draft a standard claim without a Jepson concession. If an examiner requests a Jepson format claim, the applicant may be best advised to resist the request and instead argue that a conventional claim format is sufficient and proper. Specifically, the Federal Circuit's new approach makes the risk-to-reward ratio of Jepson claims increasingly unfavorable.
Additionally, if the applicant chooses to rely on an assertion that the preamble recites something well known in the art, Xencor this decision reaffirms that the applicant has the burden to prove it. If the alleged prior art portion of a Jepson claim is not actually established in the literature or if the specification does not show that the applicant possessed it, the claim would not likely survive an examiner's rejection or a later validity challenge.
Patentees who already have Jepson claims in granted patents should evaluate their portfolios and consider whether reissue applications or other claim strategies in related pending cases might be beneficial. Other patentees and practitioners who have routinely avoided the use of Jepson claims have now had that strategy fully validated.
Footnote
1. In re Xencor, Inc., --- F.4th ---, 2025 WL 793963 (Fed. Cir. 2025).
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