Zinc Finger Nuclease and TALEN Patent Filings: Sequence Listing Requirements

Introduction

Patent applications directed to zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs) routinely contain substantial nucleotide and amino acid sequence information. That creates a practical filing issue that is easy to underestimate: when does a sequence need to appear in the patent’s formal sequence listing and how should it be prepared?

For applications filed on or after July 1, 2022, the governing framework is generally WIPO Standard ST.26, which replaced the former ST.25 standard. In the United States, the corresponding requirements are implemented through 37 C.F.R. §§ 1.831–1.835. The USPTO states that applications with a filing date – or, for national-phase applications, an international filing date – on or after July 1, 2022, that disclose qualifying nucleotide or amino acid sequences must include a compliant Sequence Listing XML. For ZFN and TALEN applications, the challenge is not simply generating an XML file. Counsel must determine which sequences require listing, how engineered sequences should be represented, how sequence features should be annotated and how the listing interacts with the application’s substantive disclosure and claim strategy.

1. Why Sequence Listings Matter in ZFN and TALEN Applications

ZFN and TALEN inventions frequently involve multiple layers of sequence information.

A single application may disclose:

Some of these sequences may be required in the formal sequence listing, while others may fall outside the listing requirement.

The distinction matters because the sequence listing is not merely an administrative attachment. For U.S. applications, the USPTO explains that the content of the required sequence-listing portion is considered part of the application’s disclosure.

Consequently, sequence-listing preparation should be treated as part of substantive patent drafting and filing review.

2. ST.26 Is the Current Standard

WIPO Standard ST.26 established a harmonized approach for presenting nucleotide and amino acid sequences in patent applications.

For applications filed on or after July 1, 2022, ST.26 replaced ST.25 for qualifying sequence disclosures. The principal change is the move from the older text-based ST.25 format to an XML-based format.

The USPTO likewise requires qualifying U.S. applications filed on or after July 1, 2022 to use ST.26-compliant XML; an applicant cannot elect to use ST.25 for a new qualifying filing.

WIPO’s current ST.26 materials are particularly important because the standard continues to evolve. WIPO’s current list identifies ST.26 Version 2.0, approved in November 2025, with entry into force scheduled for July 1, 2027.

For current filings, counsel should therefore verify the version and implementation requirements applicable on the actual filing date rather than relying on an older sequence-listing checklist.

3. What Sequences Trigger the Requirement?

The critical question is whether the application discloses a qualifying nucleotide or amino acid sequence by enumeration of its residues.

Under the PCT implementation of ST.26, qualifying sequences generally include:

Sequences below those thresholds generally should not be assigned their own sequence identification number in the sequence listing.

This threshold is particularly important for ZFN and TALEN drafting because these applications often contain short motifs, linkers, recognition segments and abbreviated sequence disclosures.

The attorney should not simply assume that every string of biological characters appearing in the specification belongs in the sequence listing.

4. ZFN Applications: What Usually Requires Attention?

A typical ZFN construct may include:

  1. zinc-finger DNA-binding domains;
  2. a linker or connector;
  3. a nuclease domain, such as a FokI-derived cleavage domain;
  4. a nuclear-localization sequence;
  5. additional fusion sequences; and
  6. a nucleic acid encoding the resulting fusion protein.

Depending on how the invention is disclosed, both nucleotide and amino acid sequences may therefore be relevant.

For example, an application might disclose:

SEQ ID NO: 1  –  nucleotide sequence encoding a zinc-finger nuclease.

and

SEQ ID NO: 2  –  amino acid sequence of the zinc-finger nuclease.

If both sequences meet the applicable ST.26 criteria, the sequence listing should represent them appropriately.

Practical drafting point

Do not assume that providing the nucleotide sequence eliminates the need to separately disclose a corresponding amino acid sequence when the amino acid sequence itself is disclosed and independently qualifies for inclusion.

The sequence-listing rules address nucleotide and amino acid sequences separately and the applicant should build the listing around the sequences actually disclosed rather than merely around the sequences that happen to be easiest to generate computationally.

5. TALEN Applications Can Be Even More Sequence-Intensive

TALEN applications frequently present a more complicated sequence-management problem because the DNA-binding portion can contain repeated TALE motifs.

A typical TALEN may include:

The application may also disclose:

The drafting team should therefore determine carefully which sequences are actually disclosed as enumerated sequences and which are described only conceptually or by reference to a structural formula or general definition.

6. Sequence Listing Is Not the Same as Sequence Disclosure

This distinction is fundamental.

A patent application can contain sequence-related information in:

The formal sequence listing is the standardized representation required for qualifying sequence disclosures.

WIPO’s PCT instructions state that a sequence required to be included in the sequence listing is one disclosed in the international application by enumeration of its residues and falling within the applicable ST.26 definitions.

Accordingly, the drafting team should conduct a whole-application sequence audit rather than simply asking the scientist to export the sequences from a laboratory database.

7. The Sequence Listing Should Be Created From the Patent Disclosure

A common workflow error is:

Scientist supplies FASTA files → paralegal converts them to XML → filing is complete.

That workflow can miss important sequences.

A better approach is:

Application disclosure → sequence inventory → classification → ST.26 generation → validation → attorney review.

The sequence inventory should identify every sequence appearing in:

The team can then determine which sequences need formal sequence identifiers and which do not.

8. Sequence Identification Numbers Must Be Consistent

Sequence identifiers should be used consistently throughout the application.

For example:

“a zinc-finger nuclease comprising the amino acid sequence of SEQ ID NO: 12”

should correspond to the correct sequence in the sequence listing.

An error in sequence numbering can create substantial downstream problems.

The attorney should therefore perform a SEQ ID NO. cross-check covering:

Older ST.25 guidance expressly required sequences represented in the listing to be referred to in the description, claims, or drawings using “SEQ ID NO:” followed by the identifier.

Under ST.26, the data structure and terminology have changed, but the practical drafting lesson remains: sequence identifiers must be traceable throughout the application.

9. Annotating ZFN and TALEN Sequences

ST.26 does more than provide a container for raw sequences. It establishes structured sequence data and controlled vocabulary for identifying relevant features.

For ZFN and TALEN applications, appropriate feature annotation can be especially useful.

Depending on the disclosure, features may correspond to:

The patent team should not invent feature annotations merely because they would be useful commercially. An annotation should accurately reflect what the application discloses and what the ST.26 vocabulary permits.

10. Engineered Sequences Require Particular Care

ZFN and TALEN inventions often involve engineered sequences rather than naturally occurring sequences.

For example, a claimed protein might comprise:

a zinc-finger DNA-binding domain having substitutions at specified residues and a heterologous nuclease domain.

The sequence-listing preparation should accurately represent the actual disclosed sequence.

Do not silently “normalize” an engineered sequence to a naturally occurring reference sequence.

Similarly, if the invention includes a family of variants, the attorney should distinguish between:

These different forms of disclosure can have different sequence-listing implications.

11. Do Not Use the Sequence Listing to Expand the Invention

One of the most important substantive precautions is that the sequence listing should accurately reflect the application as filed.

The sequence listing should not become a vehicle for introducing additional variants that were not otherwise part of the intended disclosure.

For example, suppose the specification discloses one specific TALEN amino acid sequence, but the laboratory team later supplies 200 variants.

The attorney should not automatically place all 200 variants into the sequence listing simply because they are experimentally available.

The key question is whether those sequences are part of the application’s disclosure and whether they satisfy the applicable sequence-listing requirements.

This is particularly important because the USPTO treats the required sequence-listing portion as part of the application’s disclosure.

12. Sequence Listing and Written-Description Strategy

For biotechnology inventions, sequence disclosure can be closely connected to written-description and enablement strategy.

A patent claiming:

“an isolated TALEN comprising a DNA-binding domain having at least 90% sequence identity to SEQ ID NO: 25”

may require substantially more than simply depositing SEQ ID NO: 25 into the sequence listing.

Counsel should separately consider:

A compliant sequence listing solves a format and disclosure-organization requirement; it does not independently establish written-description or enablement support.

13. ST.26 XML Requirements

For qualifying U.S. applications, the sequence listing must be provided as an XML file complying with the applicable ST.26 requirements.

The USPTO describes an ST.26 sequence listing as a separate part of the specification and states that the XML must conform to the applicable requirements of 37 C.F.R. §§ 1.831–1.835.

WIPO likewise describes ST.26 as an XML-based standard for presenting nucleotide and amino acid sequence listings.

The practical implication is straightforward:

Do not manually construct the final XML unless there is a compelling reason and the resulting file can be fully validated.

14. Use WIPO Sequence

WIPO provides WIPO Sequence, a software application designed to prepare ST.26-compliant sequence listings.

WIPO currently identifies WIPO Sequence as a tool for applicants preparing nucleotide and amino acid sequence listings for national and international applications. WIPO also provides a separate Validator for checking ST.26 compliance.

As of the current WIPO release information, WIPO Sequence version 3.1.3 was released July 28, 2026 and WIPO Sequence Validator version 3.1.2 was released June 30, 2026.

Because software versions and standards can change, the filing team should use the version applicable to the filing date and verify the final output before submission.

15. Validate Before Filing

Validation should be treated as a mandatory quality-control step.

WIPO explains that ST.26 sequence listings can be validated using WIPO Sequence Validator and that international applications may be checked for ST.26 compliance during the international search or preliminary examination process.

A practical workflow is:

  1. Generate the sequence listing.
  2. Run the validator.
  3. Correct every validation error.
  4. Review warnings separately.
  5. Compare sequence identifiers against the specification.
  6. Confirm bibliographic information.
  7. Confirm that all intended sequences are present.
  8. Confirm that unintended sequences have not been introduced.
  9. Re-run validation after every substantive change.
  10. Archive the final validated file with the filing record.

16. U.S. National Applications and PCT Applications

The filing route matters.

For U.S. national-phase applications, the USPTO states that the relevant date for determining ST.26 applicability is the PCT international filing date, not the date of U.S. national-phase entry.

For PCT applications, WIPO’s Receiving Office Guidelines state that a sequence listing forming part of the description must comply with ST.26 and be filed in XML format.

This produces an important practical rule:

Do not determine sequence-listing format solely from the date on which the U.S. national-phase application is submitted.

The international filing date can control the analysis.

17. Priority Applications and the ST.25/ST.26 Transition

The transition from ST.25 to ST.26 creates another potential trap.

For applications filed on or after July 1, 2022, the fact that an earlier priority application contained an ST.25 sequence listing does not generally allow the later application to use ST.25.

The USPTO expressly states that an application benefiting from an earlier application that contained an ST.25 listing is nevertheless required to submit a compliant ST.26 XML sequence listing when the later application’s filing date triggers the ST.26 rules.

WIPO likewise explains that the filing date – not the priority date – is the reference date for the ST.25/ST.26 transition.

This is especially important in biotechnology portfolios where a U.S. provisional, PCT application and national applications may have different filing dates.

18. Provisional Applications

U.S. provisional applications are treated somewhat differently.

The USPTO states that provisional applications are not required to contain a sequence listing. However, if an applicant chooses to submit a sequence listing with a provisional filed on or after July 1, 2022, the listing must comply with the applicable ST.26 requirements.

For ZFN and TALEN inventions, counsel should nevertheless think strategically about whether the provisional disclosure adequately supports the later claims.

A decision not to file a formal sequence listing does not eliminate the need to provide sufficient substantive sequence disclosure when the applicant expects to rely on particular sequences later.

19. Sequence Listing vs. the Specification

A sequence listing should not be viewed as a substitute for a well-drafted biotechnology specification.

For a ZFN or TALEN application, the specification should ordinarily explain the invention in sufficient technical detail, including as appropriate:

The sequence listing provides standardized machine-readable sequence data. The specification provides the technical and legal context.

Both matter.

20. Common Filing Errors

Error 1: Filing an old ST.25 listing

For a qualifying application filed on or after July 1, 2022, this is generally not the correct format.

Error 2: Omitting a qualifying sequence

A sequence embedded in an example, claim, or figure can be overlooked during sequence-listing preparation.

Error 3: Listing every short sequence

Not every short biological string necessarily qualifies for its own sequence identifier.

Error 4: Creating sequences that were not actually disclosed

The sequence listing should not be used casually to add new substantive disclosure.

Error 5: Incorrect sequence numbering

A single incorrect SEQ ID NO. can propagate through claims, specification, examples and prosecution documents.

Error 6: Failing to update the listing after an amendment

When sequences change, the sequence listing and substantive application must remain synchronized.

Error 7: Relying on unvalidated XML

A file that looks correct to a human reader may still fail ST.26 validation.

Error 8: Ignoring the PCT filing date

For national-phase cases, the relevant date can be the international filing date.

21. A Practical ZFN/TALEN Sequence-Listing Checklist

Application inventory

ST.26 analysis

ZFN/TALEN-specific review

Cross-check

Filing

Conclusion

For ZFN and TALEN patent applications, sequence-listing compliance is not a clerical afterthought. The biological complexity of these inventions makes sequence management part of the core patent-drafting process. For qualifying applications filed on or after July 1, 2022, WIPO ST.26 and its XML-based sequence-listing framework are central to the filing process. The USPTO implements the requirements through 37 C.F.R. §§ 1.831–1.835, while PCT applications are governed by the corresponding WIPO framework.

The best practice is to treat the sequence listing as a controlled patent document:

Inventory the sequences → determine which qualify → prepare the ST.26 XML → validate it → cross-check every sequence identifier → confirm substantive support → file the final version.

For ZFN and TALEN portfolios in particular, this disciplined approach can reduce filing defects, prevent sequence-number inconsistencies and help ensure that the machine-readable sequence disclosure accurately reflects the invention the patent team intends to protect.

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