A new mitochondrial DNA (mtDNA) mutation of tRNA(Leu)(UUR) at nucleotide position 3271 (MELAS3271) was determined to be involved in the pathogenic process of mitochondrial diseases MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) using intercellular transfer of patient-derived mtDNA to mtDNA-less HeLa cells (rho 0 HeLa cells). Cybrid clones containing imported mtDNA exclusively from a MELAS patient with MELAS3271 mtDNA were isolated, and the influence of MELAS3271 mtDNA on mitochondrial translation activity and mitochondrial respiratory complex I enzyme activity were examined. Accumulation of more than 87% MELAS3271 mutant mtDNA in the cybrid clones induced both low complex I activity and abnormal mtDNA-encoded polypeptide synthesis including at least complex I subunit ND6. suggesting involvement of the new MELAS-associated mutation in the pathogenesis.