We report on a 12-year-old boy with the myopathic form of phosphoglycerate kinase (PGK) deficiency, and unique kinetic and physical characteristics of the mutant enzyme (PGK North Carolina). A G-to-T substitution at the 5' end of intron 4 was identified in the PGK gene of this patient. The mutation destroys the consensus sequence GT at the 5' splice junction of the intron. Activation of a cryptic splice site within intron 4 causes the insertion into the transcript of a 30-bp fragment at the 5' end of intron 4. This insertion results in ten additional amino acids within the "nose" of the PGK molecule, but does not generate a frameshift or a premature stop codon.