To determine the pathogenicities of mutated mtDNAs in patients with respiration defects, the possible involvement of nuclear DNA mutations has to be excluded, since respiratory function is controlled by both nuclear DNA and mtDNA. This was achieved by showing that the mutated mtDNAs and respiration defects were co-transferred from patients to mtDNA-less human cells, and the resultant cybrid clones carrying mutated mtDNAs expressed respiration defects. To decide whether the cybrid clones expressed respiration defects, in this study the lowest limits of normal respiratory function were evaluated by transfer of mtDNAs from 54 normal subjects to mtDNA-less HeLa cells. The resultant cybrid clones showed that 71% respiratory function was the lowest limit of mtDNAs from normal subjects. On the other hand, cybrid clones carrying pathogenic mtDNAs from patients with mitochondrial diseases showed 0-64% respiratory function, suggesting that less than 71% respiratory function in cybrid clones should be a reliable indicator of whether the mutated mtDNAs of the patients were pathogenic.