Tryptanthrin and its analogs were shown to be anti-tumor agents both in vitro on cancer cell lines [10,11] and in vivo on rat model [12], being potent inhibitors of indoleamine 2,3-dioxygenase (IDO1) [13], tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase 2 (IDO2) [14]. The gene discussed is TDO2; the disease is cancer.