CTLA4 and Merkel cell skin cancer: Monoclonal antibodies directed against programmed cell death 1 (PD-1, e.g., nivolumab and pembrolizumab), programmed cell death ligand 1 (PD-L1, e.g., avelumab, atezolizumab, and durvalumab), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4, e.g., ipilimumab) have been approved for treating multiple solid tumor types, including melanoma, Merkel cell carcinoma, head and neck squamous cell carcinoma, lung cancer, urothelial carcinoma, renal cell carcinoma, and gastrointestinal cancers.