Genetic defects of the myophosphorylase (PYGM) gene result in McArdle's disease (GSD type V) (OMIM 232600) characterized by exercise intolerance, premature fatigue, muscle cramps, and recurrent myoglobinuria 1. Exercise intolerance generally starts in childhood, but overt episodes of muscle cramps and myoglobinuria generally develop later in adolescence. Brief isometric contraction and less intense but sustained dynamic exercise are two primary precipitants. Affected patients have mutations in both alleles of the PYGM gene. Molecular heterogeneity has been demonstrated by identification of different mutations in the coding regions or splice sites of the gene 2, 3. We describe a novel homozygous missense sequence variant (c.281G>C; p.Arg94Pro) in exon 2 in an adolescent with a rapidly progressive phenotype.