Our present and previous findings5, 6, 7 indicate that pleural tumour-originated CCL2 mobilizes two distinct cell populations from the bone marrow: mast cells and CD11b+Gr1+ cells41, 42, both known to respond to CCL2 (refs 7, 25), to facilitate breast cancer metastasis to the lungs34, and to sustain tumour growth by promoting angiogenesis28. The gene discussed is CCL2; the disease is breast cancer.