Tay-Sachs disease patients of Ashkenazi Jewish and non-Jewish French Canadian origin are affected with a clinically identical form of this inherited disease. Both have a similar gene frequency for the disorder, which is tenfold higher than that found in the general population. Unlike other patients with the disease, who often display variation at the clinical or biochemical level, the absence of such differences between these two groups has prompted the idea that they may harbor the same mutation. In this report, a complementary DNA clone coding for the alpha chain of human beta-hexosaminidase has been used to analyze the genetic lesions in the alpha-chain locus of two patients with Tay-Sachs disease from each of these groups. On the basis of DNA hybridization analyses, the alpha-chain gene of the Ashkenazi patients appears intact while the alpha-chain gene of French Canadian patients has a 5' deletion of approximately 5 to 8 kilobases.