Summary: Signal-regulatory protein alpha (SIRP-α), also known as CD172a, is a cell surface protein expressed mainly by myeloid cells such as macrophages. Its receptor, CD47, is ubiquitously expressed on the surface of all cells and has been found to be overexpressed in some cancers. CD47 binding to SIRP-α delivers a "Do Not Eat Me" signal to macrophages. In the context of a tumor, this signaling aids in tumor evasion of the immune system. Inhibitors of this interaction hold great promise for cancer immunotherapy.