The TEL/PDGFβR gene, which encodes a fusion protein containing the ETS-family member TEL fused to the protein-tyrosine kinase domain of the platelet-derived growth factor receptor-β (PDGFβR), confers interleukin 3 (IL-3)–independent growth on Ba/F3 hematopoietic cells. TEL/PDGFβR mutants have been generated that contain tyrosine-to-phenylalanine (Tyr→Phe) substitutions at phosphorylation sites present in the native PDGFβR to assess the role of these sites in cell transformation by TEL/PDGFβR. Similar to previous findings in a murine bone marrow transplantation model, full transformation of Ba/F3 cells to IL-3–independent survival and proliferation required the TEL/PDGFβR juxtamembrane and carboxy terminal phosphorylation sites. In contrast to previous reports concerning comparable mutants in the native PDGFβR, each of the TEL/PDGFβR mutants is fully active as a protein-tyrosine kinase. Expression of the TEL/PDGFβR fusion protein causes hyperphosphorylation and activation of signal transducer and activator of transcription (STAT5), and this activation of STAT5 requires the juxtamembrane Tyr579 and Tyr581 in the TEL/PDGFβR fusion. Hyperphosphosphorylation of phospholipase Cγ (PLCγ) and the p85 subunit of phosphatidylinositol 3-kinase (PI3K) requires the carboxy terminal tyrosine residues of TEL/PDGFβR. Thus, full transformation of Ba/F3 cells by TEL/PDGFβR requires engagement of PI3K and PLCγ and activation of STAT5. Taken together with the growth properties of cells transformed by the TEL/PDGFβR variants, these findings indicate that a minimal combination of these signaling intermediates contributes to hematopoietic transformation by the wild-type TEL/PDGFβR fusion.