1-Methyl-DL-tryptophan is primarily used in research settings to study the indoleamine 2,3-dioxygenase (IDO) pathway, which plays a significant role in immune regulation. It acts as an inhibitor of IDO, an enzyme involved in the metabolism of tryptophan, and is often employed to investigate its effects on immune responses, particularly in cancer immunotherapy. By blocking IDO, it helps to prevent the suppression of T-cell activity, thereby enhancing the immune system's ability to target and destroy cancer cells. Additionally, it is used in studies exploring autoimmune diseases and inflammatory conditions, where IDO modulation is of interest. Its application extends to understanding the role of tryptophan catabolism in various physiological and pathological processes.