What are the standard single-letter codes for modified and unusual nucleotides in WIPO ST.25?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

WIPO ST.25 provides standard single-letter codes for modified and unusual nucleotides in sequence listings. According to MPEP 2423:

“The nucleotides listed in Appendix 2, Table 2, are the only permitted nucleotides. Where a sequence includes “other” nucleotides, they must be represented as “n” and further described in a feature table, as provided by Appendix 2, Section 7.”

Some key codes include:

• n – any nucleotide (a, c, g, or t/u)
• r – purine (a or g)
• y – pyrimidine (c, t, or u)
• m – a or c
• k – g or t/u
• s – c or g
• w – a or t/u

For any modified or unusual nucleotides not covered by these codes, use “n” and provide additional details in the feature table.