GBE1 and glycogen storage disease due to glycogen branching enzyme deficiency: Accounting for less than 3% of all GSD diagnoses, glycogen storage disease IV (GSD4) is an autosomal recessive disorder caused by glycogen branching enzyme 1 (GBE1) deficiency, resulting in the accumulation of polyglucosan, an abnormal glycogen, in affected tissues, such as the liver, heart, skeletal muscle, and brain to varying degrees [1,[3], [4], [5], [6]].