– Phosphorylase b kinase gamma catalytic chain, skeletal muscle isoform, also known as Phosphorylase kinase subunit gamma-1 and PHKG1, is a member of the protein kinase superfamily and CAMK Ser/Thr protein kinase family. PHKG1 is the catalytic member of a 16 subunit protein kinase complex that contains equimolar ratios of 4 subunit types. The complex is a crucial glycogenolytic regulatory enzyme. Muscle glycogenosis caused by phosphorylase kinase (Phk) deficiency may lead to exercise intolerance, weakness and musculatur atrophy. The gene encoding the muscle isoform of the Phk gamma subunit (gamma M) is one of the candidate genes in which mutations responsible for this condition should be sought. Muscle-specific deficiency of Phk causes glycogen storage disease, clinically manifesting in exercise intolerance with early fatiguability, pain, cramps and occasionally myoglobinuria.