The nearly universal genetic code serves as a dictionary for translation from mRNA to amino acid.

The triplet code provides 64 (43) codons to specify the 20 amino acids.

Thus the code is degenerate: many amino acids specified by more than one codon; only tryptophan and methionine are encoded by a single codon.

In addition to codons that specify amino acid, there is one start (or initiation) codon (AUG, which also encodes methionine) and 3 stop (termination) codons.