PDCD1 and cancer: Several co-inhibitory receptors have been identified in cancer, including PD-1, CTLA-4, T-cell immunoglobulin and mucin domain 3 (Tim-3), lymphocyte-associated gene 3 (LAG3), B and T lymphocyte attenuator (BTLA), and T-cell immunoglobulin and ITIM domain (TIGIT), called immune checkpoints [109].