Although there are no randomized comparative trials demonstrating the superiority of immune checkpoint blockade over conventional chemotherapy in metastatic MCC, preliminary results are rapidly becoming promising for PD-L1/PD-1 inhibitors [57,58]; avelumab, nivolumab, and pembrolizumab are currently recommended as first-line, systemic treatment options for advanced MCC [5]. Here, PDCD1 is linked to Merkel cell skin cancer.