Disease summary: Andersen disease (GSD IV) is a rare genetic disorder of glycogen metabolism. It is caused by the deficient activity of the glycogen-branching enzyme, resulting in accumulation of abnormal glycogen in the liver, muscle, and/or other tissues. The disease is inherited as an autosomal recessive trait. Clinically, the symptoms and findings become evident in the first months of life. Such features typically include failure to thrive and hepatosplenomegaly. In such cases, the disease course is typically characterized by progressive cirrhosis and liver failure, leading to potentially life-threatening complications. In addition, several neuromuscular variants of GSD IV have been described that may be evident at birth, in late childhood, or in adulthood.