CD274 and Merkel cell skin cancer: For this review, a search of PubMed, Science.gov, and ClinicalTrials.gov databases was performed using the terms ‘immune checkpoint inhibitors’, ‘immunotherapy’, ‘anti-PD-1′, ‘anti-PDL-1′ in combination with ‘non-melanoma skin cancer’, ‘basal cell carcinoma’, ‘cutaneous squamous carcinoma’, or ‘Merkel cell carcinoma’.