PDCD1 and osteosarcoma: Recently, immunotherapy, such as adoptive cellular therapy, vaccination, and checkpoint inhibitors, has been becoming an attractive therapeutic strategy.4,5 Preclinical work has shown encouraging results of anti–programmed cell death 1 and anti–programmed cell death 1 ligand 1 blockade therapy in both an osteosarcoma model in humanized mice and the lung metastases of osteosarcoma.6,7 However, in a 2017 randomized clinical trial by Tawbi et al,8 only 1 of 22 patients (5%) with osteosarcoma had an objective response to pembrolizumab, an anti–programmed cell death 1 antibody.