CTLA4 and neoplasm: Although neoantigens from MSI-H tumors can elicit an immune response, MSI-H tumors also express checkpoint receptors (eg, PD-1, CTLA-4) at higher rates, which suppress the immune response and immune-mediated tumor lysis.3 Therefore, blocking PD-1 using anti-PD-1 antibodies can restore anti-tumor immune activity.