IDO1 and cancer: Considering the expression of IL4i1 in tumor-associated myeloid cells (44, 45, 46) (Table 1), the immunoregulatory effects of IL4i1 and the involvement of other amino acid-metabolizing enzymes such as IDO1 and Arg1 in the suppression of tumor immunosurveillance (132), it is not surprising that IL4i1 gained attention as a promising target for cancer therapies over the last years.