ABCG2 and glioblastoma: Therefore, we concurrently investigated the expression of the most-accepted GBM stemness proteins CD133 [4, 6] and Nestin [28] as well as other potential stem cell markers, based on several experimental studies, including the efflux transporter ABCG2 [14] and the transmembrane receptors CD44 [13] and CD95 [29] as well as the transcription factors ELF4 [11] and Nanog [12].