, also acid maltase deficiency disease, is an inherited metabolic disorder caused by an inborn lack of the enzyme alpha-1,4 glucosidase (lysosomal glucosidase; acid maltase), which is necessary to break down glycogen, a substance that is a source of energy for the body. This enzyme deficiency causes excess amounts of glycogen to accumulate in the lysosomes, which are structures within cells that break down waste products within the cell. This accumulation of glycogen in certain tissues, especially muscles, impairs their ability to function normally. Glycogen storage disease type 2 is a single disease continuum with variable rates of disease progression. In 2006, the U.S. Food and Drug Administration (FDA) approved the enzyme replacement therapy Myozyme as a treatment for all patients with glycogen storage disease type 2. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.