New PET imaging method tracks immunotherapeutic efficacy in mouse models Several cancer tumors grow through immunosuppression; that is, they manipulate biological systems in their microenvironments and signal to a specific set of immune cells--those that clear out aberrant cells--to stop acting. It is no wonder that immunotherapy designed to re-establish anti-tumor immunity is rapidly becoming the treatment of choice for these cancers. One natural immunosuppressive molecule that falls prey to helping cancer tumors is indoleamine-2,3-dioxygenase 1 (henceforth, IDO1). Because it is found in a broad range of cancer tumors, including those of the skin, breast, colon, lung, and blood, scientists have begun to see it as a promising therapeutic target: Suppress its activity and anti-tumor immunity should be back. But all endeavors so far have failed in phase 3 clinical trials--the stage at which a large number of people with the disease try out the optimal dose to test its true efficacy.