Classical GBM tumors are characterized by abnormally high levels of epidermal growth factor receptor, or EGFR. EGFR is a protein found on the surface of some cells that, when bound by epidermal growth factor, sends signals for the cell to keep growing. The EGFR abnormalities occur at a much lower rate in the three other GBM subtypes. However, TP53, the most frequently mutated gene in GBM, is not mutated in any of the Classical GBM tumors. TP53 is the gene for a protein that normally suppresses tumor growth. Clinically, the Classical group survived the longest of the subgroups in response to aggressive treatment.