+ 23 Runx1 enhancer activity in hematopoietic tissues of the embryo critically depends on intact Gata and Ets motifs. (A) Summary of Runx, Gata, and Ets mutated + 23 enhancer activity in mouse embryos. Data show the number and percentage of embryos in which LacZ expression was found in the indicated tissue. YS indicates expression in E8 BI or E10 hematopoietic cells. “Other” represents ectopic, nonreproducible expression outside the hematopoietic system, resulting from random integration of the constructs at or near endogenous enhancers. *Observed expression is not typical to + 23 enhancer/endogenous Runx1. **Observed expression was punctuate and faint compared with the nonmutated + 23 enhancer. §Observed expression was faint and restricted to a subset of endothelial cells. (B) Effects of Runx, Gata and Ets mutations on + 23 enhancer activity in E8 embryos. (i,ii) hsp68LacZ + 23^Runx F0 transgenic embryo. (i) Whole mount, showing expression in the YS. (ii) Section through the PAS and YS. Black arrowhead indicates LacZ expression in paired dorsal aortae; white arrowhead points at expressing cells in BI. (iii,iv) hsp68LacZ + 23^Gata F0 embryo. Arrowhead points at cells in the dorsolateral mesenchyme of the PAS that expresses LacZ. (v-vi) hsp68LacZ + 23^Ets F0 embryo. Arrowhead points at faint, punctuate LacZ expression in the wall of the dorsal aorta. (ii,iv,vi) Original magnification, ×200.