PD-1 blockade works during the effector phase to restore the immune function of T cells in the periphery that have been turned off following extended or high levels of antigen exposure, as in advanced cancer. The ligands for PD-1 can be expressed by tumor cells as well as tumor-infiltrating immune cells. PD-L1 expression on tumor cells varies by tumor type and also within a given tumor type, but appears to be particularly abundant in melanoma, non–small cell lung cancer (NSCLC), and ovarian cancer. In a recent study, PD-L1 expression on tumor cells was shown to be significantly associated with PD-1 expression on TILs, and was locally associated with PD-L2 expression when this ligand was also expressed. In the same study, tumor PD-L1 expression was the single factor most closely correlated with response to anti-PD-1 blockade, whereas PD-L1 expression on TILs was not associated with response. patient response to anti-PD-L1 blockade was strongest when PD-L1 was expressed by tumor-infiltrating immune cells. This antibody recognizing the PD-L1 binding region of PD1.