ABCG2 and glioblastoma: With the exception of Chen et al. [30] and Oberstadt et al. who investigated the methylation status of ABCB1 and ABCG2 in glioblastoma [31], studies reporting DNA methylation levels in cancer cell lines and/or clinical tumor samples focused on either ABCB1 [27, 28, 32–40] or ABCG2 [29, 41–43].