CD9 and synovial sarcoma: To further investigate the clinical utility of MCT1+CD9+ EVs for tumor monitoring, we evaluated serum expression of the MCT1+CD9+ ratio, as well as white blood cell (WBC) counts, hemoglobin (Hb) levels, and platelet (Plt) counts, in patients with SS from whom we could obtain a series of serum samples during multimodal treatment.