CTLA4 and metastatic melanoma: Anti-programmed death receptor-1 (anti-PD-1) antibodies alone or in combination with cytotoxic T-lymphocyte antigen-4 (CTLA-4) antibodies have improved the prognosis of metastatic melanoma substantially and have been shown to induce long-lasting partial or complete responses in melanoma patients.1–3 However, primary or acquired resistance against immune checkpoint inhibitors occurs in 50–60% of the patients.4 Thus, identifying predictive markers for response to immunotherapy with PD-1 or CTLA-4 antibodies can be beneficial in guiding treatment decisions.