Twenty-three different monogenic disorders, glycogen storage diseases (GSD 0a, 0b, Ia, Ib, II, IIb, IIIa, IIIb, IIIc, IV, V, VI, VII, IXa, IXb, IXc, IXd, X, XI, XII, XIII, XIV and XV), and inborn errors of metabolism are due to defects in genes encoding proteins involved in the metabolism of glycogen (GYS2, GYS1, G6PC, SLC37A4, GAA, LAMP2, AGL, GBE, PYGM, PYGL, PFKM, PHKA2, PHKB, PHKG2, PHKA1, PGAM2, LDHA, ALDOA, ENO1, PGM1 and GYG1). The gene discussed is PYGM; the disease is disorder of glycogen metabolism.