CSF1 and neoplasm: In the polyoma middle T antigen mouse model of mammary tumorigenesis (MMTV-PyMT), genetic ablation of Csf1 - a cytokine required for monocyte to macrophage differentiation – resulted in the attenuation of tumor metastasis and a reduction in multiple foci on the distal duct; however, the growth rate and incidence of primary tumors remained unchanged, suggesting that macrophages do not mediate tumor formation [21].