To attenuate the sampling issue, we ensured that all GBM tissues analyzed were homogeneous throughout the location groups in terms of tumor content and interplay from the microenvironment (Additional file 1: Figure S1A–D) and validated HES4 and DLL3 expression together with 11 other top differentially expressed candidate genes derived from microarray analysis in an independent set of 142 IDH1 wild-type GBMs (validation cohort). Here, HES4 is linked to glioblastoma.