AbstractThe mutation causing juvenile Tay‐Sachs disease (TSD) in two sibs of Lebanese‐Maronite origin is described. An mRNA‐containing extract of cultured fibroblasts obtained from one of the probands was used as a template to amplify the coding sequence of the hexoaminidase A (Hex A) α‐subunit. Sequencing of amplified cDNA fragments revealed a single alteration, guanine to adenine at nt 749 creating a G250D mutation. The mutation introduces a new recognition site for the restriction enzymeEcoRV, permitting identification of heterozygotes for this allele following PCR amplification andEcoRV digestion of exon 7 sequences from genomic DNA templates. In order to test the effect of this substitution, an in vitro mutagenized cDNA construct was introduced into a mammalian expression vector and transfected into monkey Cos‐1 cells separately or along with a β‐cDNA expression vector. When the mutant α‐cDNA was the only gene introduced into COS cells no enzymatic activity above endogenous COS cell activity was detected. Cotransfection of normal α‐cDNA and β‐cDNA followed by immunoprecipitation of human Hex A resulted in 20‐fold increase in the ratio between positive and negative (mock transfection) control values. This allowed the detection of some residual activity (12% of the positive control) when the mutant α‐cDNA replaced its wild‐type counterpart. The predicted protein environment in which the mutation occurs is compared to that of the adult‐onset Tay‐Sachs disease mutation caused by a Gly269→ Ser substitution in exon 7. We suggest that the more severe clinical phenotype linked to the Gly250→ Asp mutation may be explained by the less conservative substitution of the large, charged aspartate residue as compared to the small neutral serine, and the placement of the Gly250in a hydrophobic section