These factors help to explain why L-1MT is considered a poor physiological inhibitor of IDO in comparison to D-1MT and why D-1MT has broader clinical uses against cancers that overexpress any tryptophan catabolic enzyme, such as IDO1, IDO2 and tryptophan-2,3-dioxygenase (TDO) [32]. This evidence concerns the gene IDO2 and cancer.