Abnormality in the beta-hexosaminidase alpha gene underlying the clinical phenotype of a Lebanese patient with a juvenile form of Tay-Sachs disease has been studied. Clinical features were progressive spasticity, ataxia, and cognitive decline. The protein coding sequence of several beta-hexosaminidase alpha-chain complementary DNAs isolated by polymerase chain reaction was completely normal except for a G-to-A transition at nucleotide position 1511 within exon 13, which resulted in substitution of the normal arginine 504 (CGC) with histidine (CAC). Although the patient was from a first-cousin marriage, she was heterozygous for this mutation. The abnormality in the other allele, which is carried by the father, was not identified, except that it is neither of the two mutations responsible for the infantile Jewish Tay-Sachs disease. Biosynthetic and immunoprecipitation studies in cultured fibroblasts showed synthesis of the alpha-chain precursor, but the mature form of the alpha-subunit was not detected.