PYGM and glycogen storage disease V: A subset of glycogen storage diseases (GSD), such as GSD II (also known as Pompe disease), GSD III (also known as Cori or Forbes disease), or GSD V (also known as McArdle disease), exhibit a myopathic phenotype accompanied by muscle weakness that is caused by high glycogen content in skeletal muscle because of the absence of enzymes involved in glycogen breakdown (i.e., α-1,4-glucosidase [GAA], glycogen debranching enzyme [encoded by the AGL gene] or PYGM) (Hicks et al, 2011; van der Beek et al, 2012).