TDO2 and colorectal cancer: Since TDO2 was firstly reported to be overexpressed in glioma, where it plays a tumor promoting role, increasing numbers of studies reported that TDO2 was overexpressed in other tumors, such as breast cancer, basal cell carcinoma, melanoma, cervical cancer, and colorectal cancer, where it is also involved in tumor development [5, 10, 13, 14].