We documented the presence of a point mutation in the tRNA(Lys) gene of mitochondrial DNA (mtDNA) in various postmortem tissues from two patients with myoclonus epilepsy associated with ragged-red fibers (MERRF). The percentages of the mutant mtDNA were similar (93 to 99%) in both clinically affected and unaffected tissues, suggesting that preferential clinical involvement of certain tissues in MERRF is based not only on the variation of distribution of the mutant mtDNA, but also on other factors such as differences in the threshold in various CNS regions and organs.