Pompe disease is a rare (estimated at 1 in every 40,000 births), inherited and often fatal dis-order that disables the heart and muscles. It is caused by mutations in a gene that encodes an enzyme called alpha-glucosidase (GAA). Normally, the body uses GAA to break down glycogen, the long-term storage form of sugar, into glucose, which the body can utilize to gain energy. But in Pompe disease, mutations in the GAA gene reduce or completely eliminate the activity of this essential enzyme. Excessive amounts of glycogen accumulate everywhere in the body, but the cells of the heart and skeletal muscles are the most seriously affected. Researchers have identified up to 70 different mutations in the GAA gene that cause the symptoms of Pompe disease, which can vary widely in terms of age of onset and severity. The severity of the disease and the age of onset are related to the degree of enzyme deficiency.