KDR and breast cancer: Moreover, after injecting an MMTV-PyMT–derived breast cancer cell line, F246-6, into control Flk-1fl/fl and Csf1r-Cre+Flk-1fl/fl mice, in which Vegfr2 (Flk-1) was specifically ablated in CSF1R+ myeloid cells, we harvested the BM from Cre– and Cre+ littermates and differentiated into macrophages or MDSCs in vitro.