GAA and glycogen storage disease II: Pompe disease (OMIM 232300), also known as glycogen storage disease type 2 (GSD2) is a rare, progressive, autosomal recessive disorder caused by a deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA; OMIM 606800), leading to lysosomal glycogen accumulation in many tissues, particularly respiratory, skeletal and cardiac muscles (1–3).