CXCR4 and cancer: The stromal cell-derived factor-1 (SDF-1/CXCR4) axis has been considered as a seven-transmembrane G protein-coupled receptor which led to tumor metastasis in several cancers (Kucia et al., 2005; Müller et al., 2001; Saur et al., 2005; Zeelenberg et al., 2003; Lu et al., 2015b).