Glycogen storage disease type III (GSD3) is an inherited disorder that results from an accumulation of a complex sugar (glycogen) in many organs of the body, most particularly the liver and muscles. This is due to a defective enzyme called the glycogen debranching enzyme. There are four subtypes of GSD III; GSD IIIa, GSD IIIb, GSD IIIc, and GSD IIId. GSD IIIa and IIb are the most common. Beginning in infancy, individuals with GSD III can have low blood sugar, elevated blood levels of liver enzymes, and excess amounts of fats in the blood. Typically individuals develop an enlarged liver as they get older, which may develop into chronic liver disease and ultimately liver failure. Growth is often affected because of the liver problems. The majority of individuals affected with GSD IIIa have weakened muscles, affecting both the heart and skeletal muscles, usually appearing in early childhood and is slowly progressive. GSD IIIc and GSD IIId are extremely rare. Good dietary control can prevent most of the symptoms associated with GSD3. GSD3 is caused by pathogenic variants in the AGL gene.