A role of the chemokine/receptor pair CXCL12/CXCR4, normally controlling cell trafficking within the marrow (Nagasawa, 2000), in the development of solid tumour bone marrow metastases has been shown for small-cell lung cancer (Burger et al, 2003), breast cancer (Muller et al, 2001), and renal cell cancer (Zagzag et al, 2005). The gene discussed is CXCR4; the disease is breast cancer.