IDO1 and neoplasm: Catabolism product of tryptophan, generated through the activity of Indoleamine 2,3-dioxygenase 1 (IDO1) Labadie et al. (2019). Favors a tumor permissive microenvironment with PD-1 expression Liu et al. (2018) and CD4+ exhaustion Rad Pour et al. (2019)