How should modified nucleotides be represented in a sequence listing?

Modified nucleotides should be represented as follows:

1. Represent the modified nucleotide as the corresponding unmodified nucleotide (a, c, g, or t) whenever possible.
2. If it cannot be represented by an unmodified nucleotide, use the symbol “n”.
3. Further describe the modification in the feature table using the feature key “modified_base” and the mandatory qualifier “mod_base”.
4. Use a single abbreviation from Table 2: List of Modified Nucleotides as the qualifier value.
5. If the abbreviation is “OTHER”, provide the complete unabbreviated name in a “note” qualifier.

The MPEP states: “Any modified nucleotide in a sequence that cannot otherwise be represented by any other symbol in Table 1: List of Nucleotides Symbols (see MPEP § 2412.03(a)), i.e., an “other” nucleotide, must be represented by the symbol “n”. The symbol “n” is the equivalent of only one residue.“

To learn more:

• modified nucleotides
• feature table
• WIPO Standard ST.26
• sequence representation