HEXB and Salla disease: In Sandhoff disease (SD), the Hexb gene encoding the β-subunit of the lysosomal hydrolytic enzyme, β-hexosaminidase consisting of Hex A and Hex B subunits, is mutated and the enzyme activities of both subunits are simultaneously lost, resulting in an inability to hydrolyze GM2 ganglioside and the accumulation of GM2 mainly in neurons.