The following are the official IUB base codes:
A, C, G &T (or U)
M (A or C)
R (A or G)
W (A or T/U)
S (C or G)
Y (C or T/U)
K (G or T/U)
B (C, G or T/U)
D (A, G or T/U)
H (A, C or T/U)
V (A, C or G)
N (A, C, G or T/U)
For more information, visit http://www.chem.qmul.ac.uk/iubmb/misc/naseq.html.