Indoleamine 2,3-dioxygenase (IDO) is selected as the POI for PROTAC, because it is a Trp-catabolizing enzyme overexpressed in tumor tissues that converts Trp to kynurenine (Kyn), leading to the dysfunction of dendritic cells (DCs) and the suppression of effector T cells23. This evidence concerns the gene IDO1 and neoplasm.