IDO1 and cancer: It has been shown that IDO1, the most important enzyme that catalyzes the initial, rate-limiting step in the degradation of Trp along the kynurenine pathway, is highly expressed in many types of human cancers, including endometrial [12], colon [13], and epithelial ovarian carcinomas [14], and IDO1 is generally associated with poor prognosis [15].