A reciprocal relationship was recently found in an orthotopic GL261 cell-based tumor model, whereby IDO-competent brain tumors were infiltrated by an increased frequency of the immunosuppressive CD4+FoxP3+GITR+ regulatory T cells (Tregs), coincident with a decreased frequency of CD8+ cytotoxic T cells when compared with IDO-deficient brain tumors [24]. Here, IDO1 is linked to neoplasm.