In 2013, several groups reported PD-L1 expression on MCC tumor cells and/or PD-1 expression on TILs in the tumor microenvironment (TME), strengthening the rationale for immunotherapy agents that block the PD-1/PD-L1 axis to be used in MCC treatment (18, 50, 51). Here, CD274 is linked to Merkel cell skin cancer.