MT-ND5 and cancer: To explain the observed results, we hypothesize that, as we previously found in tumorigenesis studies on cells carrying mtDNA with heteroplasmic and homoplasmic mutations in the complex I subunit ND5 gene [30], the 4,977-bp mtDNA deletion could function in cancer development as follows: in the initial stage, when cancer cells are under stress because of a carcinogenic insult or oxidative stress damage, the deletion emerges.