Jepson Claims and the Written Description Requirement

The Federal Circuit recently issued an opinion In re Xencor, Inc., No. 2024-1870 (Fed Cir March 13, 2025), holding that the preamble of a Jepson requires the written description support from the application as filed. With that support lacking in the application at issue in the case, Xencor’s Jepson claim was rejected on written description grounds.

Jepson claims date back in 1917 to the case of Ex parte Jepson, 1917 C.D. 62, 243 O.G. 525, in which an applicant recited a claim in a form that listed basic elements known in the prior art, and then stated that the invention was “characterized by” that the pervious elements were combined with recited inventive features. The decision (by an Assistant Commissioner) referred to this form of claiming as being in accord with “common German and English practice,” which he said was “to state briefly the old general structure in the claim, followed by such an expression as ‘characterized by’ the certain particular structure which constitutes the patentee’s invention,” 1917 C.D. at 69. He ruled that there was “nothing in the law to interdict” an applicant from employing such a claim. Id. at 70.

The form for Jepson claim first made it into the rules in 1967,[1] as 37 CFR 1.75(e), which specifies the use of “a phrase such as ‘wherein the improvement comprises’” between the two parts of the claim (the old and the new). Such claims have increasingly fallen out of favor in U.S. practice, with the percentage of U.S. patents including Jepson claim language dropping from over 8% in the early 1980s to less than 0.1% in recent years.

In contrast, in Europe a related two-part claim form is required of applicants “[w]henever appropriate,” EPC Rule 43(1), with the words “characterised by” or “characterised in that” separating the old features from the new.[2] Given the pervasiveness of such a form at the EPO,[3] why has the Jepson claim fallen out of favor in the U.S.?

One problem is the prior art effect of listing an element in the preamble of a Jepson claim. In U.S. practice drafting “a claim in Jepson format … is taken as an implied admission that the subject matter of the preamble is the prior art work of another,” MPEP 2129(III), although there are certain narrow exceptions to that general rule. In contrast, in EPO practice an applicant can remove the prior art effect of listing elements in the first part of the claim by stating to the Examiner “that a mistake was made,” EPO Guidelines G-VII, 5.1. Also, the EPO approach to inventive step serves to limit the ability of an Examiner to rely on the first part of a two-part claim as prior art.

Xencor involved a Jepson claim reciting “a method of treating a patient by administering an anti-C5 antibody with an Fc domain, the improvement comprising” certain specified features of the antibody and the Fc domain.[4] The problem was that the application did not have adequate written description of the process of treating a patient any anti-C5 antibody. Did it need to? Xencor argued that the use of the Jepson claim form meant that the invention was in the second part of the claim, the part after the magic language “the improvement comprising,” and that therefore only the second part of the claim needed to meet the written description requirement of 35 USC 112(a).

The Federal Circuit panel disagreed. As it stated, “the Jepson claim invention is the totality of what is set out in the claim, just as it is for a non-Jepson claim. The 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.” Xencor at 17 (emphasis in original). Thus the rejection of Xencor’s Jepson claim was affirmed.[5]

As noted above, Jepson claims are rare in U.S. practice, so the decision is not expected to have much impact. However, it does foreclose the use of Jepson claims in an attempt to get around problems with adequate written description in an already-filed application.

[1] See 31 Fed Reg 12922 (Oct 4, 1966).

[2] PCT Rule 6.3(b) indicates a similar preference for the two-part claim form.

[3] The EPO allows deviations from the two-part form under certain circumstances. See EPO Guidelines for Examination F-IV, 2.2.

[4] The application also included a claim not in Jepson form, with a preamble reciting, “A method of treating a patient by administering an anti-C5 antibody.”

[5] The claim that was not in Jepson form was also rejected as lacking adequate written description support.