To define genetic lesions driving leukemia, we targeted cre-dependent Sleeping Beauty (SB) transposon mutagenesis to the blood-forming system using a hematopoietic-selective vav 1 oncogene (vav1) promoter. Leukemias of diverse lineages ensued, most commonly lymphoid leukemia and erythroleukemia. The inclusion of a transgenic allele of Janus kinase 2 (JAK2)V617F resulted in acceleration of transposon-driven disease and strong selection for erythroleukemic pathology with transformation of bipotential erythro-megakaryocytic cells. The genes encoding the E-twenty-six (ETS) transcription factors Ets related gene (Erg) and Ets1 were the most common sites for transposon insertion in SB-induced JAK2V617F-positive erythroleukemias, present in 87.5% and 65%, respectively, of independent leukemias examined. The role of activated Erg was validated by reproducing erythroleukemic pathology in mice transplanted with fetal liver cells expressing translocated in liposarcoma (TLS)-ERG, an activated form of ERG found in human leukemia. Via application of SB mutagenesis to TLS-ERG-induced erythroid transformation, we identified multiple loci as likely collaborators with activation of Erg. Jak2 was identified as a common transposon insertion site in TLS-ERG-induced disease, strongly validating the cooperation between JAK2V617F and transposon insertion at the Erg locus in the JAK2V617F-positive leukemias. Moreover, loci expressing other regulators of signal transduction pathways were conspicuous among the common transposon insertion sites in TLS-ERG-driven leukemia, suggesting that a key mechanism in erythroleukemia may be the collaboration of lesions disturbing erythroid maturation, most notably in genes of the ETS family, with mutations that reduce dependence on exogenous signals.