Granulocyte-colony stimulating factor (G-CSF) secreted by 4T1 mammary tumor cells has been shown to induce CD11b+Gr1+ cell expansion [17], and we, along with others, have shown that CD11b+Gr1+ cells accumulate in the spleens and lungs of mice bearing metastatic 4T1 murine mammary tumors [17–20]. The gene discussed is CSF3; the disease is breast cancer.