The table below gives you the encoding for the four bases (A, C, T, G) and for ambiguous positions in your DNA-sequence. This one-letter-code is usually used in FASTA-Files and other DNA file formats. The etymology should give you a mnemonic to memorize the codes.

Table 1. Table with FASTA-codes and their meaning.
Code Meaning Etymology Complement Opposite

A

A

Adenosine

T

B

T/U

T

Thymidine/Uridine

A

V

G

G

Guanine

C

H

C

C

Cytidine

G

D

K

G or T

Keto

M

M

M

A or C

Amino

K

K

R

A or G

Purine

Y

Y

Y

C or T

Pyrimidine

R

R

S

C or G

Strong

S

W

W

A or T

Weak

W

S

B

C or G or T

not A (B comes after A)

V

A

V

A or C or G

not T/U (V comes after U)

B

T/U

H

A or C or T

not G (H comes after G)

D

G

D

A or G or T

not C (D comes after C)

H

C

X/N

G or A or T or C

any

N

.

.

not G or A or T or C

.

N

-

gap of indeterminate length

Source: http://www.ncbi.nlm.nih.gov/blast/fasta.shtml