HEXA and Sandhoff disease: GM2-gangliosidosis includes (I) Tay-Sachs disease (TSD, OMIM 272800), at which mutations occur in the HEXA gene and only HexA activity is disrupted (variant B); (II) Sandhoff disease (SD; OMIM 268800), caused by mutations in HEXB gene, at which the activity of HexA and HexB is disrupted (variant O); and (III) GM2 activator protein deficiency (OMIM 272750), at which mutations take place in the GM2A gene (variant AB) (Mahuran, 1999).