TDO2 and melanoma: 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.