Furthermore, intense IDO expression is seen in astrocytes within a reactive gliosis at the margin of orthotopic murine glioma tumors (Figure 3), and IDO activity has been documented in TAMs from a rat orthotopic glioblastoma model using an immunohistochemical method to stain tissue for quinolinic acid, a downstream tryptophan metabolite [137]. Here, IDO1 is linked to central nervous system cancer.