NDUFA9 and squamous cell carcinoma: Under hypoxia condition, the expression of NDUFA5, NDUFS6, NDUFA9 (mitochondrial respiratory chain complex І), cytochrome B (complex III), mitochondrial-encoded subunit I (COXI), and nuclear-encoded subunit IV (COXIV) (complex IV) decreased in TE-1 cells (Esophageal squamous cell carcinoma cell line), which promoted ROS generation and further accelerated the proliferation of tumor cells [40].