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Table 1 IUPAC nomenclature for nucleic acids.

From: Computational RNA secondary structure design: empirical complexity and improved methods

Symbol Meaning Origin of designation
G G Guanine
A A Adenine
T T Thymine
C C Cytosine
R G or A puRine
Y T or C pYrimidine
M A or C aMino
K G or T Ketone
S G or C Strong interaction (3 H bonds)
W A or T Weak interaction (2 H bonds)
H A or C or T not-G, H follows G in the alphabet
B G or T or C not- A, B follows A
V G or C or A not-T (not-U), V follows U
D G or A or T not-C, D follows C
N G or A or T or C aNy