A novel missense mutation (1565G-->A) was identified in the cDNA and genomic DNA coding for the beta-hexosaminidase beta-subunit of a Japanese patient with infantile Sandhoff disease. The patient was homozygous for this mutation, which should result in a cysteine-to-tyrosine substitution at codon 522. Computer-assisted analysis of this amino acid substitution predicted alteration in the secondary structure in the region of a highly conserved sequence. An immunofluorescence study revealed the accumulation of GM2 ganglioside in cultured fibroblasts from the patient with this mutation.