KIT and breast cancer: In a murine mammary tumor model, tumor that grows in pre-irradiated mammary tissue has decreased vascular density and is more hypoxic and metastatic, recapitulating the clinical features of locally relapsed breast cancer after radiation therapy; HIF-1-dependent Kit ligand expression by hypoxic tumor cells mobilizes c-Kit+ PMN-MDSCs to home to pre-metastatic lungs to promote metastasis (194).