Therefore, genetic and pharmacologic experiments will be essential to understand the relative effects of IL4i1 versus IDO1 in a given experimental setting; this is especially significant given the limited effects of IDO1 inhibitors in cancer, which were used without (i) understanding IL4i1 was likely expressed in same environments and (ii) that regulation of ferroptosis was an unknown physiological output from both enzymes. Here, IL4I1 is linked to cancer.