CXCR4 and breast cancer: In our previous studies, by establishing and using trastuzumab-resistant breast cancer cell models, we initially found upregulation of C-X-C motif chemokine receptor 4 (CXCR4), a G protein-coupled receptor (GPCR) of stromal cell-derived factor-1 (SDF-1α; CXCL12) in trastuzumab-resistant breast cancer.