McArdle disease is caused by inherited deficiency of myophosphorylase, the skeletal muscle-specific isoform of glycogen phosphorylase (GP-MM), with preservation of the other two glycogen phosphorylase isoforms, the liver (GP-LL) and brain glycogen phosphorylase (GP-BB); as such, this defect essentially leads to myopathy with no alterations of glycogen metabolism reported in other tissues [4]. Here, PYGM is linked to glycogen storage disease V.