CYCS and neoplasm: HBV integration sites in mtDNA from tumour tissues, compared with those from non-tumour tissues (11/17 versus 0/3; P = 0.073, Fisher’s exact test), were preferentially located within mitochondrial genes related to the OXPHOS system, which is the final biochemical pathway for ATP production and is composed of five multiprotein enzyme complexes (I–V) and two electron carriers (coenzyme Q and cytochrome c)30.