Immunotherapies have revolutionized the care of many cancers, teaching the body’s own immune cells to recognize and attack tumor cells. Leading the way are drugs known as checkpoint inhibitors, which block a kind of “white flag” that tumors wave as a peace signal to T cells that would very much like to kill them. Really, this white flag is a protein called PD-L1 – many tumors coat themselves in it. When PD-L1 on the surface of a cancer cell sees its partner, PD1, on T cells, these T cells are tricked into letting the cancer cell live. To counteract this dirty trick, checkpoint inhibitor drugs block the functions of PD-L1 on tumor cells (e.g. atezolizumab) or PD1 on T cells (e.g. pembrolizumab), allowing T cells to go about their cancer-killing business.