Avoiding Sequence Listing Pitfalls Under ST.26

Sequence listings are a critical component of many biotechnology and life sciences patent applications. Under 37 C.F.R. §§ 1.821–1.825, U.S. patent applications must comply with specific sequence listing rules.

On July 1, 2022, the World Intellectual Property Organization (WIPO) implemented the Standard ST.26, replacing the long-standing Standard ST.25. Because there was no transition period, both formats remain relevant depending on the application’s filing date. Navigating this transition is essential to avoid costly errors or loss of priority.

Below are key considerations for preparing and filing sequence listings:

1. ST.25 Required
   Applications with a filing date on or before June 30, 2022, must continue using the ST.25 format throughout prosecution. Even if a sequence listing is filed after July 1, 2022, it cannot be converted to ST.26. For U.S. national phase applications, the international filing date serves as the relevant filing date for determining which standard applies.
2. ST.26 Required
   Applications with a filing date on or after July 1, 2022, must use the ST.26 format, even if they claim priority to an earlier application that used ST.25.
   
3. Continuations and Divisionals (CON/DIV)
   If a CON or DIV application is filed on or after July 1, 2022, its sequence listing must be in ST.26 format, even when the parent application was filed under ST.25 rules.
4. Best Practices
   First submit the ST.25 listing, wait for the USPTO to issue a Notice of Defective Sequence Listing, and then submit the ST.26 version. This process ensures that all parental sequence information remains on record, reducing the risk of accidental omissions during conversion.
5. Short Sequences
   ST.26 excludes sequences of fewer than 10 specifically defined nucleotides or four specifically defined amino acids. If these short sequences are included only in the original ST.25 listing, they must be added to the specification to prevent loss of subject matter, which could affect priority.
6. Wildcards
   Under ST.25, “Xaa” represents any amino acid without restriction. Under ST.26, “X” is limited to the 22 naturally occurring amino acids, a narrower interpretation that may require careful review during conversion.
7. Amino Acid Representation
   ST.25: Amino acids are shown in a three-letter uppercase format (e.g., Ala, Gly, Ser). ST.26: Amino acids are shown in a single-letter uppercase format (e.g., A, G, S).
   
8. RNA Sequences
   RNA sequences are not directly represented under ST.26. Instead, they must be written as DNA sequences and identified as RNA using the “mol_type” qualifier.
9. Double-Stranded DNA
   For double-stranded DNA, only the sense strand needs to be included, unless the antisense strand is essential to the invention. If the antisense strand is relevant, it should be provided as a separate sequence, clearly identified as antisense.

While sequence listings may seem like a formality, errors can lead to loss of subject matter or priority, impacting patent enforceability. Careful attention to the filing date, format requirements, and nuances of the ST.25 to ST.26 transition will help ensure compliance and protect valuable intellectual property rights.

This article appeared in the 2025 Life Sciences IP Tool Kit.