How does WIPO Standard ST.26 define “unknown” or “other” nucleotides and amino acids?

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.

MPEP 2412.03(a) refers to WIPO Standard ST.26, which defines “unknown” or “other” nucleotides and amino acids using specific symbols:

• For nucleotides, the symbol “n” represents “unknown” or “other”
• For amino acids, the symbol “X” represents “unknown” or “other”

The MPEP provides tables for both nucleotide and amino acid symbols, stating:

n a or c or g or t/u; “unknown” or “other”

X A or R or N or D or C or Q or E or G or H or I or L or K or M or F or P or O or S or U or T or W or Y or V; “unknown” or “other”

These symbols are not considered “specifically defined” and are used when the exact nucleotide or amino acid is unknown or not listed in the standard set.