Furthermore, studies on targeting tryptophan catabolism enzymes, such as indoleamine-2,3-dioxygenase (IDO1/IDO2), and tryptophan-2,3-dioxygenase (TDO/TDO2), which are expressed by many immune cells and solid tumors, including breast cancer are underway (26). The gene discussed is IDO2; the disease is breast cancer.