GAPDH and glioblastoma: To determine whether reference housekeeping genes would serve as surrogate markers for the total RNA yield or the total number of EVs, we analyzed the levels of GAPDH mRNA, 18S rRNA, and hsa-miR-103 in EVs isolated from 11 glioblastoma and 3 non-glioblastoma adherent cell lines (cultured under serum conditions).