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). Here, HEXB is linked to hereditary thrombophilia due to congenital protein S deficiency.