CD9 and neuroblastoma: Exosomes were further analyzed by flow cytometry in a subset of NB patients (Onset n = 9; End of Induction n = 9) using a monoclonal antibody (mAb) recognizing the typical exosomal surface marker, tetraspanin CD9 [5], to confirm their purity, and with the NB-specific anti-GD2 mAb.