ESR1 and neoplasm: Moreover, at the molecular level, high MT-CO2 protein expression was associated with elevated Ki67 (a marker of cell proliferation), the genetic amplification of several oncogenes (HER2, MYC, CCND1 and MDM2), the deletion of PTEN (a known tumor suppressor) and the down-regulation of estrogen receptor (ER-alpha) expression (85).