Ex parte Chowdhury: When Function Can Unite a Markush Group

Patent claims often need to cover several alternatives without turning each option into a separate claim. A Markush group is one familiar way to do that: the claim identifies alternatives using language such as “selected from the group consisting of.” The format is especially common in chemical, pharmaceutical, and biotechnology patents, but the underlying issue can arise whenever an invention permits interchangeable components.

The difficult question is how much the listed alternatives must resemble one another. On August 25, 2026, the U.S. Patent and Trademark Office gave applicants useful guidance when it designated Ex parte Chowdhury, Appeal 2025-002261, as an informative Patent Trial and Appeal Board decision. The Board reversed an improper-Markush-grouping rejection even though the listed microRNAs had different nucleotide sequences. In the context of the claimed invention, their shared function mattered more than those structural differences.

The decision does not make every functionally described list proper. It does show why an examiner and an applicant must analyze the alternatives in the context of the claim and specification—not in isolation.

What is a Markush claim?

A Markush claim defines a genus or subgenus by expressly listing alternative members. The familiar formulation is “a member selected from the group consisting of A, B, and C.” That structure can let one claim cover several alternatives that perform an intended role in the invention.

The USPTO’s Manual of Patent Examining Procedure § 2117 explains that an improper grouping may exist when the members lack a single structural similarity or a common use. When the group appears as part of a combination or process, the alternatives should be substitutable for one another with an expectation that the same intended result will occur.

Those standards sound straightforward until an examiner asks the wrong comparative question. Two items may be structurally different in the abstract yet interchangeable for the limited job the claim assigns them. The Board’s reasoning in Chowdhury focuses attention on that distinction.

The claimed invention used microRNAs as radiation-response markers

Ex parte Chowdhury arose from U.S. Patent Application No. 17/005,548, filed August 28, 2020, and naming Dipanjan Chowdhury and Chandan Guha as inventors. The specification concerned microRNAs, commonly called miRNAs, that can be detected in biological fluids. It described changes in serum levels of specified miRNAs following total-body irradiation and connected those changes with radiation exposure, prognosis, and the effectiveness of treatment.

Representative claim 60 recited a method of treating radiation-induced damage. In simplified terms, the method measured one or more listed miRNAs at a first time, administered treatment, measured the miRNAs again, and used an increase or decrease in specified markers to assess whether the treatment was effective and what treatment should follow.

The examiner rejected claims 60, 66, 67, 88, 90–95, and 101–104 for containing improper Markush groupings. A separate rejection under 35 U.S.C. § 101 had been withdrawn and was not the issue before the Board.

Why the examiner found the group improper

The examiner reasoned that the miRNAs had different chemical structures because each had a different nucleotide sequence. According to the rejection, their common characteristic of being composed of nucleotides was not a sufficient structural similarity. The examiner also concluded that the listed miRNAs were not part of a chemical or art-recognized class whose members would be expected, based on prior knowledge, to behave in the same manner and produce the same result.

That analysis treated the biological activity of each miRNA as the relevant function. Because individual miRNAs can behave differently within cells, the examiner viewed the claimed alternatives as insufficiently unified.

The applicants framed the inquiry differently. They argued that the miRNAs were not being claimed as compounds by themselves. They were used as markers within a treatment method. For that claimed purpose, their relevant characteristic was that their measurable levels correlated with radiation exposure and treatment response.

The PTAB focused on function in the claimed context

The Board agreed with the applicants and reversed the rejection in its February 5, 2026 decision.

The opinion drew from In re Harnisch, 631 F.2d 716 (C.C.P.A. 1980), and the Federal Circuit’s decision in Multilayer Stretch Cling Film Holdings, Inc. v. Berry Plastics Corp., 831 F.3d 1350 (Fed. Cir. 2016). The Board explained that when the listed members constitute a subgenus and the specification describes them as useful for the invention because of their similarities, placing them in a Markush group is not necessarily improper.

The controlling perspective was the role the miRNAs played in the claimed method. The claims did not depend on each miRNA performing the same underlying biological function. They depended on the miRNAs being measurable indicators of radiation exposure and treatment response. The examiner had acknowledged that the listed miRNAs were correlated with radiation exposure and were transcribed and processed in similar ways.

The Board therefore concluded that the miRNAs were interchangeable for the purpose that mattered to the invention: each could serve as a quantifiable marker. Their different nucleotide sequences and potentially different biological behavior did not defeat the grouping because those differences were not what made the claimed method work.

The diaper-fastener example explains the rule

The Board used an example already found in MPEP § 2117. A disposable-diaper claim might identify a pressure-sensitive adhesive, hook-and-loop structure, snap, or buckle as alternative fasteners. Those components are structurally quite different. Yet they may form an art-recognized class in the context of the invention because each performs the same relevant function: making the diaper repositionable and refastenable.

That analogy is helpful well beyond biotechnology. A proper analysis does not ask only whether A, B, and C look alike. It asks what the claim requires A, B, and C to do, whether the specification supports that shared role, and whether a skilled person would expect the alternatives to achieve the intended result in that context.

For the Chowdhury claims, the relevant similarity was not that every miRNA had the same sequence or performed the same cellular task. It was that the identified miRNAs could be quantified and used as indicators of the patient’s radiation response.

Why the “informative” designation matters

The PTAB issued the decision on February 5, 2026, but the USPTO did not designate it informative until August 25, following a stakeholder nomination. The timing matters because the designation signals that the agency considers the reasoning useful guidance for recurring issues.

An informative decision is not the same as a precedential decision. The USPTO explains that precedential opinions establish binding Board authority, while informative opinions provide Board norms and guidance on recurring or developing issues. Applicants should therefore treat Chowdhury as influential guidance, not as a substitute for the governing cases, statutes, regulations, and complete prosecution record.

Its practical value is nevertheless significant. An applicant facing an improper-Markush-grouping rejection now has a recent, designated PTAB decision applying the USPTO’s own MPEP framework to structurally different biological alternatives.

What applicants should build into the specification

Chowdhury is as much a lesson in application drafting as in Office Action response strategy. The successful argument depended on the specification identifying the listed miRNAs as a coherent set and explaining why they mattered to the claimed method.

When an application may use a Markush group, the specification should do more than provide a long inventory of alternatives. It should describe the genus or subgenus the inventor understands those alternatives to form, the role each member plays in the claimed combination or process, and the reason a skilled person would expect substitution to preserve the intended result.

That support can become crucial if an examiner focuses on differences among the alternatives. A well-developed specification gives the applicant a record for explaining why those differences are immaterial to the claimed relationship.

Tucker Law’s patent-application practice approaches claim scope and specification support together. Broad claim language is most defensible when the disclosure explains why the claimed alternatives belong together.

How to respond to an improper Markush grouping rejection

A strong response should first identify the examiner’s theory precisely. Is the rejection based on the absence of structural similarity, lack of common use, lack of expected substitutability, or several of those points? The applicant can then connect each challenged element to the actual function required by the claim.

The response should ground that function in the specification and explain why a skilled person would understand the listed members as a subgenus or art-recognized class for that purpose. If the examiner is analyzing each member’s broader properties instead of its claimed role, Chowdhury supplies a useful illustration of why context can change the result.

Applicants should also distinguish a Markush issue from other possible grounds. Even a proper grouping may still face written-description, enablement, definiteness, anticipation, obviousness, restriction, or election requirements. Reversing an improper-grouping rejection does not resolve those separate questions.

If prosecution reaches an impasse, an appeal to the Patent Trial and Appeal Board may be appropriate. The appeal record should show not only that the alternatives share a feature, but why that feature makes them interchangeable in the claimed invention.

The larger lesson for claim strategy

Ex parte Chowdhury does not eliminate the limits on Markush claiming. Instead, it sharpens the inquiry. Structural divergence alone may not be decisive when the alternatives perform a common, specification-supported function in the claimed context.

That principle can help applicants preserve commercially meaningful scope. Splitting every alternative into separate claims can increase cost, complicate prosecution, and make it harder to protect the functional breadth of an invention. But a broad grouping must rest on a disclosure that explains its unity.

Inventors and businesses developing patent portfolios should consider that unity before filing, not only after a rejection arrives. Tucker Law assists clients with patent drafting, prosecution, and intellectual-property strategy. If your application involves alternative compounds, biomarkers, components, or process steps, contact Tucker Law to evaluate whether the claim structure and specification support the scope the business needs.

This article provides general legal information and is not legal advice. Patentability and claim-drafting decisions depend on the particular invention, disclosure, claims, prior art, and prosecution record.