Indoleamine 2,3-dioxygenase (IDO) and the closely related enzyme tryptophan 2,3-dioxygenase (TDO) utilize tryptophan as the substrate in the first reaction toward generating kynurenine, a molecule suppressive to T-cells that plays a role in both cancer cell immune evasion and preventing rejection during pregnancy. This evidence concerns the gene IDO1 and cancer.