Tumor cells and recruited immunosuppressive cells release cytokines such as interleukin (IL)-10, transforming growth factor–beta (TGF-β), IL-4, and the enzyme indoleamine 2,3-dioxygenase 1 (IDO-1) that contribute a cytokine milieu that further suppresses infiltrated immune cells [20] (Fig. 1B). Here, IDO1 is linked to neoplasm.