It was not until recently that the role of TDO in cancer was appreciated, when emerging evidence showed that TDO, but not IDO1, is the central Trp-degrading enzyme in human glioma cells, and that the TDO–AhR pathway was associated with malignant progression and poor survival for this tumor (Opitz et al., 2011). The gene discussed is TDO2; the disease is central nervous system cancer.