IDO1 and melanoma: For example, clinical trials that involve using the inhibition of indoleamine 2,3-dioxygenase 1 (IDO1), a tryptophan-degrading enzyme, in combination with blockade of the PD1 pathway in patients with melanoma did not show improved treatment efficacy compared with PD1 blockade alone, which underlies an incomplete understanding of the biological role of IDO1 in cancer immunotherapy [184].