TDO2 and neoplasm: In regard to TDO in the tumor, although the mechanisms to regulate the TDO expression and its separate role in maintaining Trp homeostasis are all unclear as yet, TDO could be regarded as a candidate after resistance to IDO1 inhibitors, as well as the circulating level of Trp may be an indicator to evaluate the efficacy of inhibitors of TDO in tumor immunotherapy.