 Cysteine has traditionally been considered to be a hydrophilic amino acid, based largely on the chemical parallel between its sulfhydryl group and the hydroxyl groups in the side chains of other polar amino acids.However, the cysteine side chain has been shown to stabilize hydrophobic interactions in micelles to a greater degree than the side chain in the nonpolar amino acid glycine and the polar amino acid serine.In a statistical analysis of the frequency with which amino acids appear in different chemical environments in the structures of proteins, free cysteine residues were found to associate with hydrophobic regions of proteins.