IDO1 and neoplasm: However, the presence of TIM commonly hinders the function of immune cells.[22, 23] In solid tumors, endogenous indoleamine 2,3‐dioxygenase 1(IDO1) is highly expressed and involved in tumor immunosuppressive microenvironment (TIM).[24, 25] The overexpressed IDO1 can catalyze the decomposition of tryptophan (Trp) to kynurenine (Kyn).