Numerous studies have confirmed that IDO expression may promote the evasion of cancer cells from immunological surveillance.14 This may give tumors a survival advantage.15 Increasing expression of IDO exerts two main effects on the tumor microenvironment: depletion of tryptophan (Trp) and accumulation of toxic kynurenine (Kyn) metabolites. The gene discussed is IDO1; the disease is neoplasm.