CD274 and Merkel cell skin cancer: Therapeutic antibodies that target PD-1 and its ligand PD-L1 have been approved for treating NSCLC, melanoma, Hodgkin’s lymphoma (HL), transitional cell carcinoma (TCC), renal cell carcinoma (RCC), breast cancer (BC), Merkel cell carcinoma (MCC), hepatocellular carcinoma (HCC), head and neck squamous cell carcinoma (HNSCC), gastric cancer (GC), and many other cancers [3,4].