IDO1 and cancer: Self-renewing CSC express high levels of indoleamine 2,3-dioxygenase 1 (IDO1) and TGFβ from a variety of cancer cell lines, both of which are key inducers of Treg cell recruitment and generation (Wainwright et al., 2012; Stapelberg et al., 2014; Nakano et al., 2019; Shidal et al., 2019; Ozawa et al., 2020).