Indolemine 2, 3-dioxygenase (IDO) is a cytosolic monomeric hemoprotein enzyme that catalyses tryptophan, the least available essential amino acid in the human body, to N-formylkynurenine, which in turn rapidly degrades to give kynurenine. IDO is expressed in different tissues, especially and prominently in some subsets of antigen presenting cells (APCs) of lymphoid organs and also in the placen...