Two mutations in tRNA(Lys) gene of mitochondrial DNA were detected as the causes of this disease. We reviewed our previous studies and the recent literatures. We analyzed the mtDNA nucleotide sequence of a MERRF patient, the original case of MERRF described by Fukuhara et al., and identified a point mutation of 8,344 in tRNA(Lys) gene. This mutation detected in all 8 MERRF patients from 6 independent families, and not detected in 15 controls by polymerase chain reaction using a mismatched primer. We also quantitated the degrees of heteroplasmy of the point mutation at nt 8,344 of tRNA(Lys) in various postmortem tissues from two patients with MERRF. The percentages of the mutant mtDNA were similar in both clinically affected and unaffected tissues.