IDO1 and neoplasm: Consequently, it is no wonder that tumor cells acquire multiple mechanisms to evade immune responses, such as elimination and/or aberration of tumor antigens/MHC class I molecules, secretion of immunosuppressive cytokines such as transforming growth factor β (TGF-ß) and interleukins, recruitment of immunosuppressive immune cells (e.g., CD4+ CD25+ regulatory T cells and myeloid-derived suppressor cells), or expression of indolamine-2,3-dioxygenase (IDO) (Kaufman and Disis 2004; Munn and Mellor 2007; Garcia-Lora et al. 2003).