GM2 gangliosidosis is an autosomal recessive lysosomal disorder caused by a biallelic mutation in HEXA or HEXB, which causes a deficiency of N-acetyl-P-hexosaminidase causing cognitive impairment, psychiatric symptoms, cerebellar ataxia, seizures, macular “cherry red” spots, lower motor neuron symptoms, and hepatosplenomegaly. The gene discussed is HEXB; the disease is cerebellar ataxia.