Glycogen storage disease type IX (GSD IX) is a common form of glycogenosis due to mutations in PHKA1, PHKA2, or PHKB and PHKG2 genes resulting in the deficiency of phosphorylase kinase.
Until today, only a few cases of myopathy associated with muscle PhK deficiency caused by a mutation in the gene encoding the alpha subunit of phosphorylase kinase (PHKA1) have been reported.