PDCD1 and cancer: PD-1 is an immune checkpoint receptor expressed in cytotoxic T cells, which binds to PD-L1 in order to activate the inhibitory signal transduction in T cells, thereby mediating the immune escape of tumor cells.[18] Pembrolizumab, a highly selective PD-1 inhibitor, has been approved by the FDA for the treatment of various cancers, including malignant melanoma, non-small cell lung cancer, and Hodgkin lymphoma.