Multiple sequence alignments of representative β-hexosaminidase proteins. The sequences were aligned with Muscle . The eight motifs known to be a signature for GH20 family are shaded in grey. Residues involved in substrate binding are blue and shaded in grey. The solid arrowheads indicated the amino acid residues that form the catalytic triad in human HEXB [5, 6]. Tyrosine and alanine residues that play an important role in dimerization of two subunits in human HEXB  are white in black background. Cysteines involved in the formation of disulfide bonds in human HEXB  are red.