Normally, cytosolic glycogen branching enzyme (GBE1) associated with glycogen granules transfers terminal alpha(1,4) glucose blocks to form alpha(1,6) branches on growing glycogen molecules of both liver and muscle types. In the absence of GBE1 activity, abnormal amylopectin-like glycogen with longer alpha(1,4) chains and fewer branch points forms in all tissues where glycogen is normally found. Presentation of the disease is clinically heterogeneous: missense and nonsense mutations associated with little or no enzyme activity can lead to progressive liver disease or neuromuscuolar disease (Bao et al. 1996; Bruno et al. 2004).