Glycogen storage disease type II (Pompe's disease) is an autosomal recessive lysosomal storage disease caused by a deficiency of acid α-1,4-glucosidase (GAA; acid maltase, EC; 3.2.1.20/3), which is a key enzyme in hydrolyzation of lysosomal glycogen to glucose. The gene discussed is GAA; the disease is lysosomal storage disease.