What Are SNP Ambiguity Codes and What Do They Mean?

That's a good question.  In fact one that I had myself.  Here's what I found:

Apparently single nucleotide polymorphism (SNP) ambiguity codes were constructed by the International Union of Pure and Applied Chemistry (IUPAC) to denote nucleotide changes in SNPs.  Here is a table of the meaning of each code taken from the ENSEMBLE SNPView website.

IUPAC Code	Mnemonic	Meaning	Complement
A	Adenine	A	T
C	Cytosine	C	G
G	Guanine	G	C
T/U	Thymidine	T	A
K	Keto	G or T	M
M	Amino	A or C	K
S	Strong	C or G	S
W	Weak	A or T	W
R	Purine	A or G	Y
Y	Pyrimidine	C or T	R
B	not A	C, G, or T	V
D	not C	A, G, or T	H
H	not G	A, C, or T	D
V	not T or U	A, C, or G	B
N	any	G, A, T, or C	N