Mutations in the muscle isoform of glycogen phosphorylase (PYGM) are associated with McArdle disease (glycogen storage disease type V). More than 65 mutations in the PYGM gene that lead to McArdle disease have been identified to date.Gautron et al. (1987) isolated muscle phosphorylase cDNA clones from a human cDNA library. Northern blot experiments revealed 1 specific mRNA of 3.4 kb found uniquely in tissues expressing muscle phosphorylase.

The muscle glycogen phosphorylase protein comprises 842 amino acids (Kubisch et al., 1998).Burke et al. (1987) determined the intron/exon structure of the PYGM gene. Kubisch et al. (1998) provided a revised genomic structure for the PYGM gene, which contains 20 exons.