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USEARCH v11

IUPAC codes for sequence wildcard letters

Nucleotide
Symbol  Meaning
    
A  Adenine
     C  Cytosine
     G  Guanine
     T  Thymine
     U  Uracil
     M  A or C
     R  A or G
     W  A or T
     S  C or G
     Y  C or T
     K  G or T
     V  A or C or G
     H  A or C or T
     D  A or G or T
     B  C or G or T
     X  G or A or T or C
     N  G or A or T or C

Protein
Symbol Meaning
     X Any amino acid
     B N or D
     Z Q or E

Reference
Cornish-Bowden (1985), IUPAC-IUB symbols for nucleotide nomenclature, Nucl. Acids Res. 13: 3021-3030.