IDO1 and neoplasm: Along with inactive heme-Fe3+ being reduced into active heme-Fe2+, IDO1 catalyzes the oxidative cleavage of Trp to produce the intermediate product N-formylkynurenine, which is further hydrolyzed to Kyn. By using Trp depletion and Kyn production, IDO1 is considered as an immunomodulatory enzyme involved in anti-inflammation, tumor immune escape, and immunoregulation to promote maternal tolerance toward the allogeneic fetus, suppressing transplant rejection, regulating autoimmune disorders, and so on.