Defects in enzymes involved in glycogen metabolism result in glycogen storage diseases (GSDs), which may affect the skeletal and sometimes also the cardiac muscle. The most frequent abnormality causing GSDs is glycogen storage, whereas other and uncommon forms of GSD are due to a perturbation of the branching structure of glycogen. These latter GSDs are characterized by an accumulation of polyglucosan (PG),(1) an abnormal polysaccharide with few branched points and excessively long peripheral chains. PG is accumulated in PG bodies that can be easily identified in muscle by their typical features using histopathologic (strong periodic acid-Schiff [PAS] reaction, resistance to diastase digestion) and ultrastructural analyses.