The TGFβ growth factor family member BMP4 is a potent suppressor of breast cancer metastasis. In the mouse, the development of highly metastatic mammary tumors is associated with an accumulation of myeloid-derived suppressor cells (MDSC), the numbers of which are reduced by exogenous BMP4 expression. MDSCs are undetectable in naïve mice but can be induced by treatment with granulocyte colony-stimulating factor (G-CSF/Csf3) or by secretion of G-CSF from the tumor. Both tumor-induced and G-CSF-induced MDSCs effectively suppress T-cell activation and proliferation, leading to metastatic enhancement. BMP4 reduces the expression and secretion of G-CSF by inhibiting NF-κB (Nfkb1) activity in human and mouse tumor lines. Because MDSCs correlate with poor prognosis in patients with breast cancer, therapies based on activation of BMP4 signaling may offer a novel treatment strategy for breast cancer. Cancer Res; 74(18); 5091-102. ©2014 AACR.