CD274 and head and neck squamous cell carcinoma: Exosome PD‐L1 expression, but not the soluble PD‐L1 level, is associated with disease progression in patients with head and neck squamous cell carcinomas (HNSCCs), and the circulating PD‐L1+ TDE level appears to be a useful metric of disease and immune activity in HNSCC patients.114 Metastatic melanomas release TDEs that express PD‐L1 on their surface.