PDCD1 and Merkel cell skin cancer: Despite the promising anticancer activity offered by PD-1 and PD-L1 inhibitors, only a fraction of patients exhibit dramatic responses to single-agent anti-PD-L1/PD-1 antibodies treatment, mostly ranging from 15 to 35% depending on the individual's indication (exceptions include microsatellite-instable tumors, Merkel cell carcinoma and Hodgkin lymphoma, for which objective response rates are 50–80%).