SIRPA and cancer: CD47 is a transmembrane cell surface molecule that functions through the monocyte and macrophage receptor signal-regulatory protein alpha (SIRPα) leading to inhibition of phagocytosis and, thus, serves as a ‘don’t eat me signal’ to macrophages of the immune system38,39, and the CD47-SIRPα immune checkpoint plays a broad role in cancer immune evasion38,40.