In a clinical case report, patients whose Merkel cell carcinomas showed resistance to programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) blockade and were subsequently treated with both an HDAC inhibitor and an immune checkpoint inhibitor had increased intratumoral cytotoxic T cell infiltration following the combination therapy [26]. Here, PDCD1 is linked to Merkel cell skin cancer.