The CXCR-4/CXCL-12 axis plays a significant role in breast cancer bone metastasis, where CXCR-4 activation on the surface of breast cancer cells facilitates their adhesion to bone marrow endothelial cells, and it guides them to the bone due to the attraction of CXCL-12 produced by bone marrow stromal cells and osteoblasts. The gene discussed is CXCR4; the disease is breast cancer.