Tay-Sachs disease is a fatal genetic disorder in which harmful quantities of a fatty substance called ganglioside GM2 accumulate in the nerve cells in the brain. Infants with Tay-Sachs disease appear to develop normally for the first few months of life. Then, as nerve cells become distended with fatty material, a relentless deterioration of mental and physical abilities occurs. The child becomes blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in. A much rarer form of the disorder which occurs in patients in their twenties and early thirties is characterized by unsteadiness of gait and progressive neurological deterioration. Patients with Tay-Sachs have a "cherry-red" spot in the back of their eyes. The condition is caused by insufficient activity of an enzyme called hexosaminidase A that catalyzes the biodegradation of acidic fatty materials known as gangliosides. Gangliosides are made and biodegraded rapidly in early life as the brain develops. Patients and carriers of Tay-Sachs disease can be identified by a simple blood test that measures hexosaminidase A activity. Both parents must be carriers in order to have an affected child. When both parents are found to carry a genetic mutation in hexosaminidase A, there is a 25 percent chance with each pregnancy that the child will be affected with Tay-Sachs disease. Prenatal monitoring of pregnancies is available if desired.