CXCR4 and neoplasm: Particularly, CXCR4 expression on breast tumor cells can be regulated by several factors, such as hypoxia, vascular endothelial growth factor (VEGF), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), estrogen, transforming growth factor-beta 1 (TGF-β1), and IFN-γ, which have been shown to upregulate this receptor in tumor microenvironment [28–30].