Recently, independent clinical trials with immune checkpoint blockade-based immunotherapies utilizing avelumab and pembrolizumab that target programmed cell death 1 by PD-1 and its ligand, PD-L1, resulted in clinical response in approximately 30–60% of advanced MCC cases based on RECIST criteria for partial or complete response [47,48]. Here, PDCD1 is linked to Merkel cell skin cancer.