Aside from the influence on the immunological alterations of tumor cells, the abnormally activated indoleamine 2,3-dioxygenase (IDO1)/tryptophan 2,3-dioxygenase (TDO) pathway of tryptophan metabolism alters the immune profile of TME, which is highly related to an unoptimistic prognosis of patients with melanoma. This evidence concerns the gene IDO1 and neoplasm.