The concentrations of Trp and Kyn and the Kyn/Trp ratio have been previously used to reflect the total activity of IDO1, IDO2, and TDO.6 The Kyn/Trp ratio has been used to evaluate the prognosis of lung cancer, endometrial cancer, and ovarian cancer.46,47 This is the first time that the serum Kyn/Trp ratio has been positively correlated with the pathologic grades of gliomas. Here, TDO2 is linked to lung cancer.