RRM2B and mitochondrial DNA depletion syndrome: In particular OXPHOS activity was down regulated in p53-/- cells.[24] It has been clearly shown that the reduction in OXPHOS activity was not due to mutation in mtDNA.[24] A recent study suggested that mutations in p53R2, encoding a subunit of ribonuclease reductase (RR) cause mtDNA depletion syndrome in human.[36] We therefore asked whether p53 regulated mtDNA level in MEF cells.