PDCD1 and neoplasm: Several immune checkpoint signaling pathways are especially essential in the tumor immune microenvironment, consisting of programmed death receptors and their ligands such as PD-L1 and PD-L2, as well as those which are stimulatory (e.g., CD40L and CD70) and inhibitory (e.g., PD-1, CTLA-4, LAG-3, and TIM-3) [59].