The interaction between the chemokine CXCR4 and its ligand, CXCL12, is linked to cell proliferation, survival, invasion, and metastasis in various forms of cancer, such as breast cancer, where the upregulation of CXCR4 in cancer tissue is also accompanied by peak expression of its ligand CXCX12, at sites of common metastasis (Müller et al., 2001). The gene discussed is CXCL12; the disease is cancer.