IDO1 and neoplasm: With the nomenclature of myeloid derived suppressor cells (MDSCs) we identify a heterogeneous population of immature and mature cells of myeloid origin able to elicit T-cell anergy, promoting tumour immune-escape, via several mechanisms that include depletion of tryptophan, arginine, and cysteine due to the high expression level respectively of 2,3 indoleamine dioxygenase (IDO-1) and arginase (Arg-1), nytrosylation of T-cell receptor, and increased production and release of reactive oxygen species (ROS) [1,2,3].