Although CXCL12 signaling has been implicated in breast cancer metastasis as a homing mechanism for cancer cells to the metastatic sites, not much is currently known about the role of CXCL12 signaling in the early steps of metastasis within the primary tumor expressing CXCR7 and the mechanisms by which CXCR7-mediates breast cancer growth and metastasis. The gene discussed is CXCL12; the disease is breast cancer.