PDCD1 and cancer: The highest antitumor activities of single-agent PD-1 blockade therapy have been observed in carcinogen-induced cancers or malignancies driven by viral infections, such as classic Hodgkin's lymphoma (cHL), the virally induced Merkel cell carcinoma of the skin, microsatellite-instability high (MSI-H) cancers and desmoplastic melanoma, leading to response rates that can reach 50 to 80% [38].