Original sentence: A baby with Tay-Sachs disease appears normal at birth and seems to develop normally until about six months of age.By about two years of age, most children experience recurrent seizures and diminishing mental function.By the time a child with Tay-Sachs disease is three or four years old, the nervous system is so badly affected that life itself cannot be supported.Even with the best of care, all children with classical Tay-Sachs disease die early in childhood, usually by the age of five.Eventually, the child becomes blind, mentally retarded, paralyzed, and non-responsive to his or her environment.Initially, development slows, there is a loss of peripheral vision, and the child exhibits an abnormal startle response.Other symptoms include increasing loss of coordination, progressive inability to swallow and breathing difficulties.Tay-Sachs disease is caused by the absence of a vital enzyme called hexosaminidase A (Hex-A).The destructive process begins in the fetus early in pregnancy, although the disease is not clinically apparent until the child is several months old.The first signs of TSD can vary and are evident at different ages in affected children.The infant gradually regresses, losing skills one by one, and is eventually unable to crawl, turn over, sit, or reach out.This ongoing accumulation causes progressive damage to the cells.causes of tay-sachs diseaseWithout Hex-A, a fatty substance or lipid called GM2 ganglioside accumulates abnormally in cells, especially in the nerve cells of the brain