PDCD1 and cancer: In the context of human cancer therapy, the currently most studied ICIs are PD1 (programmed cell death protein 1), CTLA-4 (cytotoxic T lymphocyte protein 4), LAG-3 (lymphocyte activation gene 3 protein), and, TIM-3 (T cell immunoglobulin and mucin-domain containing)–for an overview including the respective ligands, see also Figure 2.