MKI67 and esophageal squamous cell carcinoma: Inhibition of cell proliferation; increase in radio-sensitivity of ESCC; enhanced irradiation-induced DNA damage, apoptosis, G2/M cell cycle arrest; increase in irradiation-induced ROS generation by suppressing Nrf2 and target genes HO-1 and NQO-1; in vivo suppression of tumor growth and expression of Ki-67 and PCNA; more profound in combination with irradiation