IDO1 and melanoma: For example, melanoma cells may also overcome immune system control through the production of negative mediators as transforming growth factor (TGF)-β, the activation of metabolic pathways such as either indolamine 2,3-dioxygenase (IDO) or CD39/CD73 axis and, lastly, the overexpression of negative immune checkpoint receptors by T-cells and related ligands (23).