The preamble is not free text

How to read the ledger below. Each numbered point carries the kind of evidence behind it. ESTABLISHED is stated in the authority's own text, with a locator. IMPLEMENTED is the rule as the authority actually applied it. OBSERVED is carried by a source outside this one. ASSUMED is the drafter's inference, flagged as such and not the authority speaking. Quotations are checked against a cached copy of the primary source by script.

In one paragraph

The Federal Circuit affirmed the Patent Trial and Appeal Board in In re Xencor on 13 March 2025 and held that the limiting preamble of a Jepson claim must itself be supported by written description, with the applicant carrying that burden and the amount of support needed varying with what a person of ordinary skill in the art already knew. Reciting prior art in a preamble therefore brings that recitation inside the specification support obligation rather than leaving it outside.

Reader summary

A Jepson claim recites the prior art in its preamble and then claims the improvement. The form is a concession: the preamble says, in effect, this much was already known. Xencor's position was that a concession of that kind does not need written-description support, because it describes what others already had.

The Federal Circuit disagreed. A Jepson preamble is limiting by construction, and a limitation needs support. The court put the burden squarely on the applicant and made it travel to the preamble, adding that how much support is enough varies with what a person of ordinary skill already knew.

The practical shape is counterintuitive, and it is what makes this record worth carrying. A drafter reaches for a Jepson claim to narrow around known art, and that move imports the recited art into the specification's obligations. The concession is not costless.

Why this matters when you are drafting

A continuation's specification is frozen; its claims are not. That asymmetry is the whole risk here. Adopting Jepson form in a continuation adds a limiting preamble to a disclosure written before anyone contemplated it, and claim 2 says the support burden follows.

A limiting preamble is a limitation for support purposes, full stop. Claim 4 separates the two questions worth asking at the drafting desk: is this preamble limiting, and if so is it described? The first is construction and the second is section 112(a), and a preamble that survives the first must clear the second.

The concession does not exempt the recitation. The intuition that you need not describe what you admit is prior art is precisely what failed. If your improvement is defined against a recited baseline, the baseline is part of what the specification must convey possession of.

Sufficiency is calibrated to the skilled reader. Claim 7. Where the recited art is genuinely standard, the burden is lighter, which means the argument to make is about what the field knew, supported by evidence, rather than about whether the burden applies at all.

What the authority establishes

1. [ESTABLISHED, the holding] "We hold that the limiting preamble of a Jepson claim must be supported with sufficient written description, and what constitutes sufficiency varies depending on the knowledge of the pertinent person of ordinary skill in the art." (P. 2.)

1. [ESTABLISHED, where the burden sits] "A patentee has the burden of providing written description; in a Jepson claim, that burden extends to the limiting preamble." (P. 2.)

1. [ESTABLISHED, the holding restated in the analysis] "We agree with the ARP that a Jepson claim preamble requires written description." (P. 16.)

1. [ESTABLISHED, the question presented] Xencor argued "that the Board and the ARP erred by construing part of one claim's preamble as limiting and requiring written description for the Jepson claim preamble of another claim". (P. 2.) Two separate preamble questions, one about whether a preamble is limiting and one about Jepson form specifically.

1. [ESTABLISHED, the Office's position, which prevailed] The Director's position was "that the preamble should be read as a whole, with all parts limiting and requiring written description", and that neither the specification nor the extrinsic evidence established sufficient support. (P. 2.)

1. [ESTABLISHED, the posture] The appeal came from an Appeals Review Panel "of the Patent Trial and Appeal Board ... rejecting the claims of its patent application as unpatentable for lack of written description". (P. 2.) This is prosecution, not litigation, so the holding governs what an examiner may require.

1. [IMPLEMENTED, sufficiency is relative, not absolute] Claim 1's second half does real work: the standard is not a fixed quantum of disclosure but what a person of ordinary skill already knew. A preamble reciting genuinely well-known art needs less; one reciting something the field would not recognize needs more.

1. [ASSUMED, drafter's inference, flagged as such] Claims 1 and 2 together make Jepson form a net cost in support terms for a continuation whose specification was written years earlier. The opinion does not discuss continuations; this follows from the burden attaching to a preamble the drafter adds later to a specification that is already fixed.

Quotations, verified against the source

All from the Federal Circuit's opinion. Page locators computed from the docket stamp.

• "We hold that the limiting preamble of a Jepson claim must be supported with sufficient written description, and what constitutes sufficiency varies depending on the knowledge of the pertinent person of ordinary skill in the art" (P. 2.)
• "A patentee has the burden of providing written description; in a Jepson claim, that burden extends to the limiting preamble" (P. 2.)
• "the preamble should be read as a whole, with all parts limiting and requiring written description" (P. 2.)
• "We agree with the ARP that a Jepson claim preamble requires written description" (P. 16.)

Tensions and open questions

How much less support a well-known preamble needs is not calibrated. Claim 1 makes sufficiency depend on the skilled artisan's knowledge without saying what evidence establishes it. The opinion notes the extrinsic evidence offered did not suffice, which sets a floor by example rather than by rule.

The holding is about Jepson form specifically, and about limiting preambles generally, in one opinion. Claim 4 shows two questions travelling together. A reader should not carry the Jepson rule to every preamble; the general rule is the ordinary one, that a limiting preamble needs support.

Unresolved by this record: how the rule interacts with a preamble reciting a commonly owned earlier patent. In a continuation family the recited baseline is often the applicant's own prior work. Nothing read here addresses whether that changes the analysis.

What this record does and does not cover

The statement of the holding and the Jepson-preamble analysis were read in full, because they are the reason this record exists.

What was NOT read closely, and is therefore outside what this record can support: the non-Jepson preamble construction and the technical substance of the claimed antibodies. This record cannot support any proposition about why the specific disclosure here was found insufficient on its facts, only about the rule the court applied. The Board and ARP decisions are known here only through the court's characterization.

Currency

This record was verified against its primary source on 2026-08-15. That is a fidelity check: the quotations, locators and attribution are accurate to that source. Whether the authority has since been narrowed, distinguished, abrogated or reversed has not been checked here. Confirm it still stands before relying on it.