PDCD1 and neoplasm: As an example, the PD-1 system discussed above is a significant checkpoint in tumor development [135], but it interacts with several of the major kinases including MAPK, JAK, and the protein kinase B pathway involving Pi3K [81] This potential for secondary effects on the wide array of kinases and related proteins is one of the reasons why the PD-1–tryptophan connections are pivotal in cellular function.