PDCD1 and Merkel cell skin cancer: Monoclonal antibodies against cytotoxic T-lymphocyte antigen 4 (CTLA-4), programmed cell death 1 (PD-1), and programmed cell death ligand-1 (PD-L1) are approved by the Food and Drug Administration (FDA) for treatment of cSCC, melanoma, and/or MCC (11).