In the process of investigating innate immune reactions against injected oncogenic cells in Drosophila, we noted that the homeodomain protein Engrailed, known since last century for its role in segmentation polarity in embryos, is reactivated in adult males where it considerably restricts the expected proliferation of these cells. Importantly, this effect was not induced following injection of selected pathogens or primary Drosophila embryonic cells. We show that the injection of t