ABCG2 and glioblastoma: Furthermore, no correlation of ABCG2 methylation and expression could be identified in a group of 20 GBM patients (Spearman’s rank correlation coefficient: -0.170, p = 0.474; Figure1E) and no significant difference in ABCG2 methylation of GBMs and normal brain has been measured (Mann Whitney test p = 0.051; Figure1F).