Activation of fibroblast growth factor receptor 1 (FGFR1) in mammary epithelial cells induces recruitment of macrophages to the mammary epithelium, which contributes to FGFR1-induced tumor formation. Using 3D co-culture assays in which FGFR1 was activated specifically in mammary epithelial cells, macrophages were found to associate with the epithelial structures and induce invasion of the cells into the surrounding basement membrane matrix. Further studies demonstrated that this interaction was dependent upon increased secretion of Cxcr2-binding chemokines by macrophages following exposure to FGFR1-induced soluble factors produced by the epithelium. For details see Bohrer and Schwertfeger on page 1294.