CD274 and osteosarcoma: Clinical data regarding melanoma [5], head and neck cancer [35], gastric cancer [36], non-small cell lung cancer [33,37], osteosarcoma [34], and other cancers have shown that the high level of exosomal PD-L1 is positively correlated with disease progressions, such as advanced tumor stage, large tumor size, lymph node-positive, and distal metastasis.