EGFR and glioblastoma: GBMs are further subdivided into four groups (Proneural, Neural, Classical, and Mesenchymal), mainly based on the abnormally high levels of mutated genes (i.e., EGFR is highly upregulated in >98% of Classical GBM, whereas TP53 (p53), which is most frequently mutated in Proneural GBM (50–60% of patients) is rarely mutated in Classical GBM).