Due to its role in tumor immune escape mechanisms, IDO expression has been found in tumor cells, DC cells, endothelial cells and even stromal cells in the TME, where it inhibits CTL infiltration and cell functions while inducing Treg recruitment (Uyttenhove et al., 2003; Löb et al., 2009; Sharma et al., 2010; Munn and Mellor, 2016). The gene discussed is IDO1; the disease is neoplasm.