Pompe disease (PD; OMIM 232300) is a severe metabolic myopathy caused by the deficiency of acid alpha-glucosidase (GAA; acid maltase, E.C.3.2.1.20), an enzyme responsible for breaking down glycogen to glucose within the acidic environment of lysosomes. The gene discussed is GAA; the disease is Glycogen storage disease due to acid maltase deficiency.