| Base | Name | Bases Represented | Complementary Base |
|---|---|---|---|
| A | Adenine | A | T |
| T | Thymidine | T | A |
| U | Uridine(RNA only) | U | A |
| G | Guanidine | G | C |
| C | Cytidine | C | G |
| Y | pYrimidine | C T | R |
| R | puRine | A G | Y |
| S | Strong(3Hbonds) | G C | S* |
| W | Weak(2Hbonds) | A T | W* |
| K | Keto | T/U G | M |
| M | aMino | A C | K |
| B | not A | C G T | V |
| D | not C | A G T | H |
| H | not G | A C T | D |
| V | not T/U | A C G | B |
| N | Unknown | A C G T | N |
* Thanks to Joost Kolkman at Maxygen who pointed out that revcomp(S)=S and revcomp(W)=W; in the source above (no longer online), revcomp(S) was W and vice-versa, which is is incorrect. I knew that but hadn't verified.