The GM2 gangliosidoses, Tay-Sachs disease and Sandhoff disease, are virtually clinically indistinguishable autosomal recessive lysosomal storage disorders caused by mutations in the genes encoding the α subunit (HEXA) or β subunit (HEXB), respectively, of the two β-hexosaminidase isoenzymes, hexosaminidase A (αβ) and hexosaminidase B (ββ). This evidence concerns the gene HEXB and Sandhoff disease.