A hypoxic tumor microenvironment favors up-regulation of CXCR4 and CXCL12 in monocytes, monocyte-derived macrophages, tumor-associated macrophages, endothelial cells, and cancer cells through a global regulator hypoxia-inducible factor-1 alpha (HIF-1α) [84]. This evidence concerns the gene CXCL12 and cancer.