TDO2 and neoplasm: Interestingly, R-2-hydroxyglutarate (R-2-HG), a metabolite that accumulates in isocitrate dehydrogenase (IDH)-mutant gliomas, enzymatically induces TDO in tumor-infiltrating myeloid cells, leading to AhR-dependent suppression of macrophage function and anti-tumor immunity (67).