CXCR6 and cancer: Two independent signaling loops have been clarified to be involved in hypoxia-stimulated breast cancer invasion and metastasis: (i) in C-X-C chemokine ligand 16 (CXCL16) signaling, cancer cells secrete CXCL16, which binds to C-X-C chemokine receptor type 6 (CXCR6) on mesenchymal stem cells (MSCs), and in turn MSCs secrete CXCL10, which binds to CXCR3 on cancer cells, and (ii) MSCs secrete chemokine ligand 5 (CCL5), which binds to C-C chemokine receptor type 5 (CCR5) on cancer cells, and cancer cells release colony-stimulating factor 1 (CSF1), which binds to CSF1R on MSCs87.