Glycogen storage isease type II (also called Pompe disease or acid maltase deficiency) is a rare, autosomal recessive metabolic disorder caused by a deficiency in the enzyme acid maltase (EC 3.2.1.20), which is needed to break down glycogen, a stored form of sugar used for energy. It is the only glycogen storage disease with a defect in lysosomal metabolism, and was the first glycogen storage disease to be identified, in 1932.