During homing, the CXCR4 expressed by HSCs and breast cancer cells binds to the CXC chemokine ligand 12 (CXCL12) expressed by bone marrow stromal cells, such as osteoblasts, endothelial cells, and fibroblasts, to retain HSCs or tumor cells in the bone marrow [103]. This evidence concerns the gene CXCR4 and breast cancer.