]> A protein modification that inserts or replaces a residue with a natural, non-standard encoded residue, such as N-formyl-L-methionine, L-selenocysteine, or L-pyrrolysine. natural, non-standard encoded residue MOD:00868 Origin: These are produced exclusively by modification of amino acids acylated to special tRNA before incorporation by ribosomes into proteins. For this reason, they have also been referred to as pre-translational modifications. Source: definition natural residue