Stromal specific activation of platelet-derived growth factor receptor-β (PDGFRβ) in the metastatic microenvironment promotes breast cancer metastasis to the brain. Specifically, the Fsp1-cre transgene was used to hyperactivate PDGFRβ in the mesenchymal population. A confocal image shows native tdTomato fluorescence (red) and GFAP (green) immunostaining in the brain stroma of a Fsp1-cre;Rosa26-LSL-tdTomato reporter mouse. The dual tdTomato(FSP1)/GFAP-positive cells represent a novel stromal population implicated in creating a prometastatic niche through PDGFRβ signaling. For details, see article by Thies and colleagues on page 606.