Tay-Sachs disease is a fatal disorder in children (usually by age 5) that causes a progressive degeneration of the central nervous system. It is caused by the absence of an enzyme called hexosaminidase A (or hex A). Without hex A, a fatty substance builds up on the nerve cells in the body, particularly the brain. The process begins early in pregnancy when the baby is developing, but is not apparent until several months after the birth. To date, there is no cure for Tay-Sachs. Dr. Tay and Dr. Sachs, who originally described this condition, noted that most Tay-Sachs babies were usually of eastern European Jewish origin. About one in 30 persons of Ashkenazi Jewish ancestry carries the Tay-Sachs gene.