McArdle disease is caused by pathogenic mutations in both copies of the gene (PYGM) encoding the muscle isoform of glycogen phosphorylase (GP-M), generally leading to loss of activity and inability to break down glycogen, making this source of energy unavailable to patients (Mc, 1951; Tsujino et al., 1995; Quinlivan et al., 2010; Lucia et al., 2012; Bartram et al., 1993; Dimauro et al., 2002). The gene discussed is PYGM; the disease is glycogen storage disease V.