IDO1 and neoplasm: It also has been found that idoelamine 2,3-dioxygenase (IDO) is highly expressed in monocytic-MDSC (M-MDSC) in chronic lymphocytic leukemia, which catabolizes the rate-limiting step of the kynurenine (Kyn) pathway, resulting in lower Trp levels and the accumulation of Kyn within the tumor microenvironment (TME) [14].