Interestingly, animal studies demonstrate that the genetic ablation of IDO1 by glioma cells results in decreased Treg recruitment [95] and significantly improved survival rates (150 days vs. 26.5 days in IDO1-competent GBM mice, p < 0.002) [95], pharmacological IDO1 enzyme inhibitors fail to recapitulate the same effect [96]. Here, IDO1 is linked to glioblastoma.