This is particularly relevant to tumor immunology, as the tumor microenvironment is saturated with factors that can promote the generation of iTregs, including factors that are directly produced by tumor cells such as TGF-β, IL-10, IL-35, and indoleamine 2,3-dioxygenase (IDO; Liu et al., 2007; Collison et al., 2010; Heckel et al., 2011; Wang et al., 2013). Here, IDO1 is linked to neoplasm.