IDO1 and cancer: Cells are equipped with several hemoproteins (69) and for some of them a role in cancer has been reported, including myoglobin (70), tryptophan 2,3-dioxygenase (TDO) and indoleamine-2,3-dioxygenase 1 and 2 (IDO1/2) (71–73), mitochondrial cytochromes (74, 75), cytochrome P450 (76, 77), and cyclooxygenases (78, 79).