Trp is most likely lower in the tumor because indoleamine 2,3‐dioxygenase 1 (IDO1) which is overexpressed in many different types of tumor cells causes immunosuppression via the Trp metabolic pathway.[15, 34] We also demonstrated that under pathological conditions, such as colon cancer, we can use therapeutic doses of Trp to treat tumor cells via intratumoral injection. Here, IDO1 is linked to colonic neoplasm.