STAT3 and neoplasm: The complex I catalyzes the electron transfer from NADH to ubiquinone in the first step of the mitochondrial respiratory chain, and its subunit NDUFA13, in addition to be required for electron transfer (Lu and Cao, 2008), functions as a tumor suppressor that binds to STAT3 and inhibits its transcriptional activity (Nallar et al., 2010).