HEXA and Tay-Sachs disease: Mutations in the HEXA gene encoding the α-chain of (β-hexosaminidase-A) Hex-A leads to Tay-Sachs disease or B variant, while mutations in the β-chain encompassing HEXB gene leads to deficiency of both Hex-A and Hex-B (total-Hex) causing Sandhoff disease or O variant.