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. The gene discussed is IDO1; the disease is neoplasm.