CD200 and neoplasm: Immune checkpoint proteins, including Programmed Death 1 (PD-1) and its ligands PD-L1 and PD-L2, Lymphocyte Activation Gene 3 (LAG-3), CD200, Cytotoxic T Lymphocyte Activator 4 (CTLA-4), and CD47, are involved in tumor immunology and benefit tumor growth.