TNFRSF9 and neoplasm: CD137 is a potent costimulatory receptor on activated T cells, which enhances anti-tumor immune response when CD137 levels are upregulated, while its negative feedback mechanism shifts CD137 from T cells to APC expressing CD137 ligand, forming CD137-CD137 ligand complex for degradation, accordingly, leading to immune escape.[61] The results of a study further revealed that PD-L1 is overexpressed in laryngeal squamous cell carcinoma (LSCC) and hypopharyngeal squamous cell carcinoma tissues and is expressed at higher levels in vocal and subglottic compared to supraglottic LSCC tissues.