A codon is the three-unit sequence (AUG, AGC, etc.) of mRNA nucleotides that codes for a specific amino acid. Since only twenty amino acids are commonly used and sixty-four (4 × 4 × 4) codon sequences are possible, the genetic code is described as “both degenerate and unambiguous.” Each codon codes for only one amino acid, but each amino acid may have more than one matching codon.