IDO1 and colorectal cancer: Since IDO1 inhibitors such as epacadostat cannot block the expression of IL4I1, IL4I1 may be the reason for the failure of clinical studies on ICB combined with IDO1 inhibition [27], suggest that IL4I1 blocking (such as CB-668) combined with IDO1 and immune checkpoint suppression may be a candidate pathway for colorectal cancer therapy.