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| Symbols | Possible substitutions | Origin and designation |
| R | A,G | puRine |
| Y | C,T | pYrimidine |
| M | A,C | aMino |
| K | G,T | Ketone |
| S | C,G | Strong interaction |
| W | A,T | Weak interaction |
| B | C,G,T | not-A, B follows A in alphabet |
| D | A,G,T | not-C, D follows C in alphabet |
| H | A,C,T | not-G, H follows G in alphabet |
| V | A,C,G | not-T/U, V follows U in alphabet |
| N | A,G,C,T | aNy |
