However, Llosa and colleagues recently refined these classic observations by demonstrating that several immune checkpoint ligands were upregulated in the dMMR tumor microenvironment, including PD-1, PD-L1, cytotoxic T-lymphocyte associated protein 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3) and IDO. The gene discussed is IDO1; the disease is neoplasm.