PYGM and glycogen storage disease V: McArdle disease (estimated prevalence of ∼1/140,000, with a similar sex distribution; Santalla et al., 2017) is an autosomal recessive disorder caused by pathogenic mutations in the gene (Pygm) encoding the skeletal muscle‐specific isoform of glycogen phosphorylase (myophosphorylase) (Villarreal‐Salazar et al., 2022), the enzyme that initiates glycogen breakdown in this tissue.