Pompe disease (glycogen storage disease type II, acid maltase deficiency, OMIM # 232300) is a lysosomal storage disorder caused by mutations in the acid α-glucosidase gene (GAA, EC 3.2.1.20) (Hirschhorn and Reuser 2001; van der Ploeg and Reuser 2008). The gene discussed is GAA; the disease is glycogen storage disease II.