IDO1 and neoplasm: In the tumor microenvironment or draining lymph nodes, IDO1 activity suppresses the function of T effector cells (Teff) and natural killer (NK) cells and promotes the induction and activation of T regulatory cells (Treg) and the activation, recruitment and expansion of myeloid-derived suppressor cells (MDSC) (Figure 1) [Fallarino(21, 29–36)].