Glycogen storage disease type IV (GSD-IV), also known as Andersen disease or amylopectinosis (MIM 23250), is a rare autosomal recessive disorder caused by a deficiency of glycogen branching enzyme (GBE) leading to the accumulation of amylopectin-like structures in affected tissues. The disease is extremely heterogeneous in terms of tissue involvement, age of onset and clinical manifestations. The human GBE cDNA is approximately 3-kb in length and encodes a 702-amino acid protein. The GBE amino acid sequence shows a high degree of conservation throughout species. The human GBE gene is located on chromosome 3p14 and consists of 16 exons spanning at least 118 kb of chromosomal DNA. Clinically the classic Andersen disease is a rapidly progressive disorder leading to terminal liver failure unless liver transplantation is performed. Several mutations have been reported in the GBE gene in patients with classic phenotype. Mutations in the GBE gene have also been identified in patients with the milder non-progressive hepatic form of the disease. Several other variants of GSD-IV have been reported: a variant with multi-system involvement including skeletal and cardiac muscle, nerve and liver; a juvenile polysaccharidosis with multi-system involvement but normal GBE activity; and the fatal neonatal neuromuscular form associated with a splice site mutation in the GBE gene. Other presentations include cardiomyopathy, arthrogryposis and even hydrops fetalis. Polyglucosan body disease, characterized by widespread upper and lower motor neuron lesions, can present with or without GBE deficiency indicating that different biochemical defects could result in an identical phenotype. It is evident that this disease exists in multiple forms with enzymatic and molecular heterogeneity unparalleled in the other types of glycogen storage diseases.