McArdle's disease: a rare frameshift mutation in exon 1 of the muscle glycogen phosphorylase gene.

We have previously discovered a common nonsense mutation in exon 1 of the myophosphosphorylase gene in patients with McArdle's disease, but this failed to explain all cases. We now report a second mutation (G-->TT) in one patient, also in exon 1. This mutation causes a shift in the reading frame which results in the replacement of Val15 by Phe. A further 10 amino acids of mis-sense protein are synthesised before a stop codon is reached.