CXCR4 and cancer: In solid cancers, a tumor microenvironment that is rich in reactive oxygen species (ROS) promotes the binding of the hypoxia inducible factor subunit HIF-1α to its response element (HRE) in the promoter region of CXCR4, thus critically influencing CXCR4-mediated expression and functions, and ultimately encouraging cancer metastasis (27, 28).