Only the 20 proteinogenic amino acids, i.e. amino acids corresponding to DNA/RNA codons, can be incorporated into proteins during translational synthesis. The proteins consisting of these amino acids are subjected to various post-translational modifications of their building blocks as hydroxylation, methylation, or phosphorylation. These modified amino acids matter for the biological activity of proteins and peptides excised from them. Other modifications of amino acids result from the “aging” of proteins and pathological processes.

As the number of amino acids discovered in nature is ever increasing, we prefer to avoid the term “unnatural amino acid” and thus designate all non-proteinogenic amino acids as “unusual amino acids”.

This category also includes the “non-α-amino acids”, as β-amino acids (e.g. isoserine), γ-amino acids (e.g. statine),…,ω-amino acids (e.g. ε-aminocaproic acid, 11-aminoundecaoic acid).