Given the ubiquitous expression of C/EBPβ in various subpopulations of CSF1R-expressing myeloid cells in both the normal mammary gland (Fig 1) and a mouse model of breast cancer progression (32), we hypothesized that macrophage-derived C/EBPβ may be fundamental to macrophage function in the mammary gland. This evidence concerns the gene CEBPB and breast cancer.