Oncogenic tyrosine kinase fusions play a key role in transforming hematopoietic stem and progenitor cells. Bcr/Abl (B/A) and TEL/PDGFβR (T/P) tyrosine kinase fusions were isolated from chronic myeloid leukemia and chronic myelomonocytic leukemia respectively. The constitutive activation of these fusions mainly promotes cell growth of target cells inducing myeloproliferative disease (MPD), which could be a precondition for transformation into acute leukemia. We found that these kinase fusions not only stimulate autonomous cell growth, but also specify cells’ myeloid fate by their own myeloid lineage instructive signals. We retrovirally transduced either the B/A or the T/P gene into purified murine hematopoietic progenitor and stem cells. Transducion of B/A or T/P into hematopoietic stem cells (HSCs) and common myeloid progenitors (CMPs) resulted in formation of spontaneous myeloid colony in cytokine- independent manners. Interestingly, B/A-transduced HSCs (B/A+ HSCs) spontaneously formed granulocyte/monocyte (GM) and megakaryocyte/erythroid (MegE) colonies, while T/P-transduced HSCs (T/P+ HSCs) only generated GM colonies. T/P+ HSCs did not generate MegE-related colonies even in the presence of erythropoietin or thrombopoietin, suggesting that T/P transduces GM-restricted permissive signals. Both B/A+ HSCs and T/P+ HSCs displayed markedly reduced potential for B cells in vitro and vivo. We then transduced B/A or T/P into purified common lymphoid progenitors (CLPs) to test their effect on lymphoid commitment. In a limiting dilution assay, 1 in 7 control-GFP transduced CLPs generated B cell progeny in vitro, while only 1 in 150 B/A+ CLP and 1 in 500 T/P+ CLPs differentiated into B cells. Instead, surprisingly, the majority of B/A+ and T/P+ CLPs differentiated into granulocytes and monocytes in vitro and in vivo. These results indicate that B/A and T/P can specify the myeloid fate counteracting the lymphoid differentiation. We then tried to locate the responsible signaling domain for growth-stimulating or instructive signals by utilizing a series of tyrosine-to-phenylalanine mutants of T/P at the major autophosphorylation sites of PDGFβR. The substitution of only two juxtamembrane tyrosine residues resulted in impairment of cytokine-independent cell growth of HSCs. In contrast, CLPs transduced with mutant T/P substituted at all eight major tyrosine residues were still reprogrammed to differentiate into the myeloid lineage. These data collectively suggest that oncogenic tyrosine kinase fusions can specify the leukemic phenotype by their own instructive signals, which use independent signaling domain for growth promoting ones.