Beta-hexosaminidase (HEX) cleaves the terminal N-acetyl galactosamine (GalNAc) from glycosaminoglycans (GAGs) and any other molecules containing a terminal GalNAc. There are two forms of HEX; HEXA and B. The A form is a trimer of the subunits alpha, beta A and beta B. The B form is a tetramer of 2 beta A and 2 beta B subunits (O'Dowd et al. 1988). Defects in the two subunits cause lysosomal storage diseases marked by the accumulation of GM2 gangliosides in neuronal cells. Defects in the alpha subunits are the cause of GM2-gangliosidosis type 1 (GM2G1) (MIM:272800), also known as Tay-Sachs disease (Okada & O'Brien 1969, Nakano et al. 1988). Classical Tay-Sachs disease is characterised by infant-onset neurodegeneration followed by paralysis, dementia and blindness, Death occurs by the age of 2 or 3 (Okada et al. 1971). The two other forms of Tay-Sachs disease, juvenile- and adult-onset, are less commom and severe than the infant-onset form (Suzuki et al. 1970, Johnson et al. 1982).