| Nucleic acid code | Common display | Meaning | Mnemonic |
|---|---|---|---|
| A | A | A | Adenine |
| C | C | C | Cytosine |
| G | G | G | Guanine |
| T (or U) | T | T (or U) | Thymine (or Uracil) |
| R | A/G | A or G | Purine |
| Y | C/T | C or T (or U) | Pyrimidines |
| K | G/T | G or T (or U) | Bases which are ketones |
| M | A/C | A or C | Bases with amino groups |
| S | C/G | C or G | Strong interaction |
| W | A/T | A or T (or U) | Weak interaction |
| B | C/G/T | Not A (i.e. C, G, T or U) | B comes after A |
| D | A/G/T | Not C (i.e. A, G, T or U) | D comes after C |
| H | A/C/T | Not G (i.e. A, C, T or U) | H comes after G |
| V | A/C/G | Neither T nor U (i.e. A, C or G) | V comes after U |
| N | A/C/G/T | A or C or G or T (or U) | Nucleic acid |
| . or - | - | Gap of indeterminate length | |