The epidermal growth factor receptor (EGFR) gene is frequently amplified and rearranged in malignant gliomas with expression of oncogenic deletion mutants (DM). The most common mutant EGFRvIII, which contains a deletion of exons 2 - 7, is constitutively autophosphorylated and inefficiently downregulated. Other less common EGFR mRNA species in gliomas contain tandem duplication of exons, which encode the tyrosine kinase (TK) and calcium mediated receptor internalization (CAIN) domains of the molecule. We examined a panel of human malignant gliomas and found one tumor that expressed four related EGFR proteins, including 125-, 140-, 170-, and 180-kDa species. This tumor also contained four EGFR-related mRNA species, including both wild type EGFR and EGFRvIII transcripts. A third transcript contained a deletion of exons 2 - 7 and 12 - 13 corresponding to the 125-kDa protein. A fourth transcript contained an in-frame, tandem duplication of exons 2 - 7 (EGFR.TDM/2 - 7). The 180-kDa, tandem duplication mutant (TDM) exhibited enhanced basal phosphorylation and impaired downregulation. In contrast to the 140-kDa EGFRvIII; however, phosphorylation of the 180-kDa EGFR.TDM/2 - 7 was strongly induced by ligand. Expression of both deletion and tandem duplication mutants in the same tumor suggests that the mechanisms responsible for DM and TDM formation might be closely related. Oncogene (2000) 19, 4542 - 4548.