TDO has also been regarded as a logical candidate to mediate resistance to IDO1-selective inhibition besides its direct roles in cancer, the expression of TDO was detected in lots of cancer types such as HCC, glioblastoma, bladder carcinoma, pancreatic carcinoma, colon carcinoma, melanoma and breast cancer[56, 123–125]. This evidence concerns the gene TDO2 and breast cancer.