Key enzymes of tryptophan metabolism, such as indoleamine-2,3-dioxygenase 1, tryptophan-2,3-dioxygenase, and kynurenine monooxygenase, play essential roles in pathophysiological processes and have been found to be overexpressed in melanoma, ovarian cancer, breast cancer, and colon cancer [6]. Here, TDO2 is linked to colonic neoplasm.