CXCR4 and breast carcinoma: Mechanistically, they found that activation of HIF signaling in osteoprogenitor cells increased the production of C-X-C motif chemokine ligand 12 (CXCL12), which leads to an increase in proliferation and dissemination of breast cancer cells through direct activation of C-X-C motif chemokine receptor 4 (CXCR4), a receptor for CXCL12.