Within the group of inherited muscle disorders, RM occurs most frequently in disorders of muscle metabolism (most commonly due to mutations in ACADVL, CPT2 and PYGM), mitochondrial dysfunction, muscular dystrophies (most commonly seen in BMD and LGMD2I), congenital myopathies where intracellular calcium homeostasis and excitation-contraction coupling are affected (RYR1) and LPIN1. Awareness of specific genotype-phenotype correlations in genetic conditions associated with RM is of importance to inform the diagnostic approach. The gene discussed is PYGM; the disease is congenital myopathy.