![embryoid body blood island embryoid body blood island](https://www.cell.com/cms/attachment/600544/4728764/gr1.jpg)
Using electrophoretic mobility shift assay, chromatin immunoprecipitation, transcriptional assays, and mutagenesis, we demonstrate that Etv2 binds to the Lmo2 enhancer and transactivates its expression. Forced overexpression of Etv2 during embryoid body differentiation induces the hematopoietic and the endothelial lineages, and transcriptional profiling in this context identifies Lmo2 as a downstream target. In addition, these cells do not form hematopoietic colonies, indicating an essential role of Etv2 in the specification of the hematopoietic lineage. In the Etv2 mutant yolk sac, Tie2(+) cells are present but do not express hematopoietic or endothelial markers. The hematopoietic activity in the E7.75 yolk sac was exclusively localized to the Etv2-EYFP(+) population. In the Etv2-EYFP transgenic mouse, the EYFP reporter is activated in the nascent mesoderm, expressed in the endothelial and blood progenitors, and in the Tie2(+), c-kit(+), and CD41(+) hematopoietic population. Both the hematopoietic and the endothelial lineages are absent in the Etv2 mutant yolk sac. In this study, we define the role of Etv2 in yolk sac blood island development using the Etv2 mutant and a novel Etv2-EYFP reporter transgenic line. We have previously reported that an Ets variant gene 2 (Etv2/ER71) mutant embryo lacks hematopoietic and endothelial lineages however, the precise roles of Etv2 in yolk sac development remains unclear. During embryogenesis, the endothelial and the hematopoietic lineages first appear during gastrulation in the blood island of the yolk sac.