PDCD1 and Merkel cell skin cancer: In particular, checkpoint blockade strategies targeting cytotoxic T-lymphocyte associated protein 4 (CTLA-4) or the programmed cell death protein 1/programmed cell death ligand-1 (PD-1/PD-L1) axis are used in many different cancers including melanoma, non-small lung cancer (NSCLC), renal cell carcinoma, bladder cancer, Hodgkin lymphoma, head and neck cancer, squamous cell carcinoma and Merkel cell carcinoma, among others.