A very rare benign inborn error of glycogen metabolism with characteristic of exercise intolerance. The disease starts generally in adolescence or adulthood. Patients may present with exercise intolerance with myalgia, cramps, fatigue, and sometimes myoglobinuria. In some cases, patients may present with progressive muscle weakness. Phosphorylase kinase (PhK) is an enzyme which plays a key role in the regulation of glycogenolysis as it is required for glycogen phosphorylase activation. It consists of four copies of each four subunits (alpha, beta, gamma and calmodulin) encoded by different genes on different chromosomes and differentially expressed in various tissues.