IDO1 and neoplasm: In the past years, the studies on tumor immune escape mechanisms led to the identification of indoleamine 2,3-dioxygenase (IDO) enzymes (IDO1 and 2), responsible of tryptophan (TRP) degradation and kynurenines (KYN) production, as key factors in tumor induced immunosuppression by direct and indirect effects on T cells [13,14].