The development and neoplastic progression of human astrocytic tumors appears to result through an accumulation of genetic alterations occurring in a relatively defined order. One such alteration is amplification of the epidermal growth factor receptor (EGFR) gene. This episomal amplification occurs in 40-50% of glioblastomas, which also normally express endogenous receptors. Moreover, a significant fraction of amplified genes are rearranged to specifically eliminate a DNA fragment containing exons 2-7 of the gene, resulting in an in-frame deletion of 801 bp of the coding sequence of the extracellular domain. Here we used retroviral transfer of such a mutant receptor (de 2-7 EGFR) into glioblastoma cells expressing normal endogenous receptors to test whether the mutant receptor was able to augment their growth and malignancy. Western blotting analysis showed that these cells expressed endogenous EGFR of 170 kDa as well as the exogenous de 2-7 EGFR of 140-155 kDa. Although holo-EGFRs were phosphorylated on tyrosine residues only after exposure of the cells to ligand, de 2-7 EGFRs were constitutively phosphorylated. In tissue culture neither addition of EGF nor expression of the mutant EGFR affected the rate of cell growth. However, when cells expressing mutant EGFR were implanted into nude mice subcutaneously or intracerebrally, tumorigenic capacity was greatly enhanced. These results suggest that a tumor-specific alteration of the EGFR plays a significant role in tumor progression perhaps by influencing interactions of tumor cells with their microenvironment in ways not easily assayed in vitro.