An arbitrary sequence using IUB codes can be specified in (5).
Example:
| Code | Base | Comment |
| G | G | Guanine |
| A | A | Adenine |
| T | T | Thymine |
| C | C | Cytosine |
| R | A, G | puRines |
| Y | C, T | pYrimidines |
| M | A, C | aMino |
| K | G, T | Keto |
| S | C, G | Strong |
| W | A, T | Weak |
| H | A, C, T | not G |
| B | C, G, T | not A |
| V | A, C, G | not T |
| D | A, G, T | not C |
| N | A, C, G, T | any |