PDCD1 and Merkel cell skin cancer: Since the only current FDA-approved MCC therapies target the PD-1/PD-L1 (programmed cell death protein 1/PD1 ligand) immune checkpoint pathway, which benefits ~50% of MCC patients (73, 75, 76, 78, 327), a syngeneic model would be a tremendous benefit and provide a much-needed immunotherapy pre-clinical model.