On the other hand, the inefficient killing of malignant cells is mostly due to a direct effect of melanoma cells through the over-production of negative modulators of immune cell [10] including adenosine, tumor necrosis factor-β (TGF-β), vascular endothelial growth factor (VEGF) and indolamine 2,3-dioxygenase (IDO) [11, 12] as well as by the loss of both class I and II antigens of the major histocompatibility complex (MHC). Here, IDO1 is linked to melanoma.