Epidermal growth factor receptor (EGFR) gene amplification is the most common genetic alteration in high-grade glioma, and approximately 50% of EGFR-amplified tumors also harbor a constitutively active mutant form of the receptor, DeltaEGFR. Although DeltaEGFR greatly enhances tumor growth and is thus an attractive target for anti-glioma therapies, recent clinical experiences with EGFR kinase inhibitors have been disappointing, because resistance is common and tumors eventually recur. Interestingly, it has not been established whether DeltaEGFR is required for maintenance of glioma growth in vivo, and, by extension, if it truly represents a rational therapeutic target. Here, we demonstrate that in vivo silencing of regulatable DeltaEGFR with doxycycline attenuates glioma growth and, therefore, that it is crucial for maintenance of enhanced tumorigenicity. Similar to the clinical experience, tumors eventually regained aggressive growth after a period of stasis, but interestingly, without re-expression of DeltaEGFR. To determine how tumors acquired this ability, we found that a unique gene, KLHDC8, herein referred to as SDeltaE (Substitute for DeltaEGFR Expression)-1, is highly expressed in these tumors, which have escaped dependence on DeltaEGFR. SDeltaE-1 is also expressed in human gliomas and knockdown of its expression in DeltaEGFR-independent "escaper" tumors suppressed tumor growth. Taken together, we conclude that DeltaEGFR is required for both glioma establishment and maintenance, and that gliomas undergo selective pressure in vivo to employ alternative compensatory pathways to maintain aggressiveness in the event of EGFR silencing. Such alternative pathways function as substitutes for DeltaEGFR signaling and should therefore be considered as potential targets for additional therapy.