Asparagine, an amino acid with a small polar side chain, was found to be the most mutable of the amino acids.[7] Cysteine and tryptophan were found to be the least mutable amino acids.[7] The side chains for cysteine and tryptophan have less common structures: cysteine's side chain contains sulfur which participates in disulfide bonds with other cysteine molecules, and tryptophan's side chain is large and aromatic.[3] Since there are several small polar amino acids, these extremes suggest that amino acids are more likely to acceptably mutate if their physical and chemical properties are more common among alternative amino acids.[6][4]