Tay-Sachs disease, the prototype of the GM2 gangliosidoses, is a catastrophic neurodegenerative disorder of infancy. The disease is caused by mutations in the HEXA gene resulting in an absence of the lysosomal enzyme, beta-hexosaminidase A. As a consequence of the enzyme deficiency, GM2 ganglioside accumulates progressively, beginning early in fetal life, to excessive amounts in the central nervous system. Rapid mental and motor deterioration starting in the first year of life leads to death by 2-4 years of age. Through the targeted disruption of the mouse Hexa gene in embryonic stem cells, we have produced mice with biochemical and neuropathologic features of Tay-Sachs disease. The mutant mice displayed < 1% of normal beta-hexosaminidase A activity and accumulated GM2 ganglioside in their central nervous system in an age-dependent manner. The accumulated ganglioside was stored in neurons as membranous cytoplasmic bodies characteristically found in the neurons of Tay-Sachs disease patients. At 3-5 months of age, the mutant mice showed no apparent defects in motor or memory function. These beta-hexosaminidase A-deficient mice should be useful for devising strategies to introduce functional enzyme and genes into the central nervous system. This model may also be valuable for studying the biochemical and pathologic changes occurring during the course of the disease.