PYGM and glycogen storage disease V: McArdle disease (glycogen storage disease V; myophosphorylase deficiency; OMIM database number ♯232600; ORPHA: 368) is an autosomal recessive disorder caused by pathogenic mutations in the gene (PYGM) encoding the muscle isoform of glycogen phosphorylase (GP-M, also known as myophosphorylase)1,2.