Nucleotide / Nucleoside / DNA Base degeneracy / redundancy table showing single letter abbreviations for combinations of bases. Names below are the free bases: ending becomes -idine or -osine for nucleosides.

A = Adenine

C = Cytosine

T = Thymine

G = Guanine

U = Uracil

N = A, G, C or T = aNy

R = A or G = puRine (9-ring with 4 N and 5 C)

Y = C or T (also U) = pYrimidine (6-ring with N at positions 1 and 3)

M = A or C = aroMatic

S =  G or C 

W = A or T 

 K = G or T = Keto 

V = A, C or G = Not T (letter after)

D = A, G or T = Not C

H = A, C or T = Not G

B = C, G or T = Not A

Each individual oligonucleotide has only one of the nucleotides at any position! A pool of nucleotides, for example as synthesized, will have all the alternatives - and hence the 'or' written above may be replaced by '+'. The degeneracies are also used to indicate uncertainty, e.g. in sequencing results. Note that difference sequence analysis programmes handle the degeneracy letters differently, sometimes ignoring them, sometimes handling them correctly. When writing by hand, it is convention to use lower-case for 'g' to avoid confusion with 'C'.

Pat Heslop-Harrison June 2004