Amino Acid Codes Reference
Look up one-letter and three-letter codes for the 20 standard amino acids and find out what ambiguous, unknown, and rare residue codes mean.
Standard amino acid codes
One-letter codes can be written together, as in ACD. For three-letter input, separate the codes with spaces, as in Ala Cys Asp.
| One-letter | Three-letter | Amino acid |
|---|---|---|
| A | Ala | Alanine |
| C | Cys | Cysteine |
| D | Asp | Aspartic acid |
| E | Glu | Glutamic acid |
| F | Phe | Phenylalanine |
| G | Gly | Glycine |
| H | His | Histidine |
| I | Ile | Isoleucine |
| K | Lys | Lysine |
| L | Leu | Leucine |
| M | Met | Methionine |
| N | Asn | Asparagine |
| P | Pro | Proline |
| Q | Gln | Glutamine |
| R | Arg | Arginine |
| S | Ser | Serine |
| T | Thr | Threonine |
| V | Val | Valine |
| W | Trp | Tryptophan |
| Y | Tyr | Tyrosine |
Ambiguous, unknown, and rare residue codes
The converter reports these codes at their input positions. It requires a sequence made up of the 20 standard codes before returning a result.
| One-letter | Three-letter | Meaning |
|---|---|---|
| B | Asx | Ambiguous code for aspartic acid or asparagine. |
| J | Xle | Ambiguous code for leucine or isoleucine. |
| Z | Glx | Ambiguous code for glutamic acid or glutamine. |
| X | Xaa | Unknown or unspecified amino acid. |
| U | Sec | Selenocysteine is outside the 20 supported amino acids. |
| O | Pyl | Pyrrolysine is outside the 20 supported amino acids. |