CXCR4 and breast cancer: CXCR4 plays an important role in lung cancer metastasis, which is a G protein-coupled receptor (GPCR) consisting of 352 amino acids [19], expressed in a variety of cells, selectively binding to its unique ligand CXCL12, also known as stromal cell-derived factor 1 (SDF-1).The binding of CXCL12 to CXCR4 induces intracellular signal transduction through multiple pathways, and plays an important role in cell growth, proliferation, migration and metastasis of breast cancer [20], ovarian cancer [21], colorectal cancer [22] and other tumors.