TDO is still necessary to determine whether IDO selective inhibitors are sufficient, as TDO, which catalyzes the same reaction as IDOs, is up-regulated in a variety of tumors, such as breast cancer, cervical cancer, glioblastoma, colorectal cancer, bladder cancer, lung cancer, and hepatocellular carcinoma, promoting immunosuppression and improving tumor invasiveness [77,78]. Here, IDO1 is linked to colorectal cancer.