Classical Tay-Sachs disease is an inherited, genetic disorder that causes progressive degeneration and destruction of the central nervous system in affected individuals. Babies born with Tay-Sachs disease appear normal at birth, and symptoms of the disease do not appear until the infants are about four to six months of age. It is at this time that these children begin to lose previously attained skills, such as sitting up or rolling over. They gradually lose their sight, hearing and swallowing abilities. There is severe developmental delay. These children usually die by the age of four.

Individuals with Tay-Sachs disease lack a substance in their body called hexosaminidase A (Hex A). Hex A is responsible for breaking down a certain type of fat called GM2-ganglioside. When Hex A is missing from the body, it cannot break down this fat. The fatty substance accumulates to toxic levels in the body, mainly in the brain and nervous system.

An estimated one in every 25 Ashkenazi Jews is a carrier for Tay-Sachs disease. The gene is located on chromosome 15.

There is another form of Tay-Sachs disease called late-onset Tay-Sachs. The cause of this disease is a significant reduction of Hex A. There is about 5% to 15% residual Hex A activity in late-onset Tay-Sachs, compared to the classical infantile form where the enzyme activity is completely absent. The later onset of the disease and slower progression of neurologic symptoms are due to a slower accumulation of GM2-ganglioside.