"We report here that both known type II topoisomerases Top2a and Top2b are present in mammalian mitochondria with especially Top2b regulating the supercoiling state of mtDNA. Loss of Top2b or its inhibition by ciprofloxacin results in accumulation of positively supercoiled mtDNA followed by cessation of mitochondrial transcription and replication initiation causing depletion of mtDNA copy number."xsd:string