Determining the role of PD-1/PD-L1, PD-L2, indoleamine 2,3-dioxygenase (IDO), sialic acid-binding immunoglobulin-like lectins (Siglecs), and CTLA-4 in animal models of IBD, patients with IBD, as well as in CRC can provide insights into disease initiation and progression that may assist in identification of novel targets. The gene discussed is IDO1; the disease is inflammatory bowel disease.