Definitions in Patent Specifications Must Be Given Controlling Weight When Clearly Expressed

In Alnylam Pharmaceuticals, Inc. v. Moderna, Inc., the Federal Circuit held definitions in patent specifications must be given controlling weight when clearly expressed. The court reasoned that once the high threshold for lexicography is satisfied, a high threshold would have to be satisfied to change that controlling definition.

Alnylam Pharmaceuticals sued Moderna over its use of SM-102 in the COVID-19 vaccine SPIKEVAX®, alleging infringement of two patents (U.S. Patent Nos. 11,246,933 and 11,382,979) that claimed cationic lipids with branched alkyl groups in their hydrophobic tails. The district court adopted Moderna’s claim construction and found no infringement; Alnylam appealed.

The Federal Circuit affirmed, holding that Alnylam had clearly defined the term “branched alkyl” in its specification, and the district court’s interpretation properly followed that express definition. The court explained:

A patentee may choose to be his own lexicographer and use terms in a manner other than their ordinary meaning, as long as the special definition of the term is clearly stated in the patent specification or file history.

The court determined that Alnylam acted as its own lexicographer in its requirement of a carbon bound to at least three other carbons. The court found the patentee’s language in the “Definitions” section of the specification to be controlling lexicography:

Unless otherwise specified, the term “branched alkyl” . . . refer[s] to an alkyl . . . group in which one carbon atom in the group (1) is bound to at least three other carbon atoms and (2) is not a ring atom of a cyclic group.

Alnylam argued that this should not apply to the claims at issue. However, the court disagreed:

The sentence in question appears under the title “Definitions.” . . . The term to be defined, “branched alkyl,” is set off in quotation marks . . . [and] uses the term “refer to,” which generally “conveys an intent . . . to be definitional.”

Alnylam attempted to invoke the “unless otherwise specified” caveat to support a broader interpretation that would encompass secondary carbons at the alpha position. The court rejected this argument holding that nothing in the claims, specification, or prosecution history provided a different definition.

Alnylam also relied on a prosecution response suggesting a broader interpretation of “branched alkyl,” but the court found this insufficient to override the clear definitional statement in the specification. The court explained: 

Moreover, as a general matter, once the high threshold for lexicography is met in a patent, it makes sense that a high threshold would have to be met before finding a departure from that controlling definition.

Strategic and Doctrinal Implications

The Federal Circuit’s opinion in Alnylam v. Moderna is a reminder that clear lexicography governs, and overriding a definition in the specification requires clarity equal to the definition it seeks to override, i.e., a high threshold. Alnylam’s argument failed because it submitted a definition for a claim term that conflicted with the definition provided in its own specification. The Federal Circuit’s opinion is a cautionary tale for both patent drafters and litigators: definitions govern, and catch-all language in of itself will not retroactively rewrite claim language. Strategic observations include the following:

• The decision reinforces the importance of patent clarity and public reliance on definitional language, echoing the court’s long-held priority of public notice over post hoc litigation strategies.
• Phrases like “unless otherwise specified” are not a license for ambiguity—they demand precision in pointing out exceptions. When combined with lexicography, such clauses invite narrow judicial interpretations unless accompanied by detailed alternative definitions.
• The court reaffirmed that the presence of embodiments in a specification that are not claimed does not justify broadening claim construction to accommodate them.

Practice Tips for Patent Litigators

1. Don’t Overreach Beyond Express Lexicography
   • Courts are unlikely to credit attorney argument over a specification’s clearly defined terms, even if broader interpretations might capture the alleged infringing product.
2. Be Wary of “Unless Otherwise Specified” Clauses
   • This language may sound flexible, but the bar for “otherwise specifying” is high—requiring clear, affirmative evidence in the intrinsic record if it contradicts a definition in the specification.
3. Prepare for Lexicography-Driven Constructions Early
   • During early claim construction phases, identify and stress definitional passages in specifications—both your own and your adversary’s. These may drive case-dispositive stipulations.
4. Map Infringement Theories to Specific Claim Terms
   • Ensure that the technical facts map tightly to the legal claim language post-claim construction.
5. Use Dependent Claims to Your Advantage—or Be Ready to Explain Them
   • Alnylam tried to argue claim breadth by invoking narrower dependent claims, but the court found those did not require a broader construction of the independent claims.

Strategic Guidance for Patent Applicants and Owners

1. Draft Definitions With Litigation in Mind
   • Definitional statements—especially those using “refer to” and placed in a “Definitions” section—will likely be binding. Be precise and consider foreseeable infringement scenarios.
2. Avoid Ambiguity in “Escape Hatch” Clauses
   • The phrase “unless otherwise specified” invites scrutiny. If used, you must clearly and explicitly point to exceptions elsewhere in the specification. Otherwise, the general rule stands.
3. Align Embodiments With Claimed Terminology
   • If embodiments are meant to illustrate a claim term, say so. Silence leaves the door open for courts to disregard those examples as non-illuminative.
4. Use Multiple Definitions if Necessary
   • If a term is intended to cover different meanings depending on context, define each variant separately, and make clear where each applies.
5. Coordinate With Prosecution Counsel on Structural Limitations
   • Prosecution counsel should avoid statements that might narrow scope unintentionally—or too broadly.
6. Beware of Generic “Catch-All” Examples
   • Including broad or generic structures as “representative” examples will not likely broaden claim scope if those examples don't meet claim limitations.

is a partner in Manatt, Phelps and Phillip’s Intellectual Property Protection and Enforcement business unit and is the author of Patent Prosecution: Law, Practice, and Procedure, 2024 Edition, and Constructing and Deconstructing Patents (2d Edition 2016).

Alnylam Pharmaceuticals, Inc. v. Moderna, Inc., 138 F.4th 1326, 2025 USPQ2d 822, 2025 WL 1572001 (Fed, Cir. 2025).

Id., 138 F.4 at 1333.

Id., 138 F.4th at 1329 (quoting U.S. Patent 11,246,933, col. 412, lines 13–17).

Id., 138 F.4 at 1333.

Id., 138 F.4 at 1334.