Diagram summarizing the findings described by Nagahata et al. The authors compared the gene expression programs of phagocytic cells from mice with those from a tunicate (Ciona), sponge (Amphimedon), and the single-celled metazoan precursor Capsaspora, revealing the selective expression of CEBPα homologs. They also found that in nonphagocytic mouse hematopoietic cells, Cebpa is repressed by the Polycomb complex. A representation of a circulating macrophage from mice is shown on the bottom. The image is also used for a generic representation of phagocytes from the more primitive species. Professional illustration by Patrick Lane, ScEYEnce Studios.