The + 23 CNE shows Runx1-related enhancer activity in hematopoietic tissues of the mouse embryo. (A) Summary of + 23 enhancer activity in lacZ-expressing F0 hsp68LacZ + 23 transgenic embryos. Data show the number and percentage of embryos in which LacZ expression was found in the indicated tissue. “Other” represents ectopic, nonreproducible expression outside the hematopoietic system, presumably resulting from random integration of the constructs at or near endogenous enhancers. (B) LacZ expression in early E8 (5-6 sp) hsp68LacZ + 23 F0 transgenic (i-iv) and Runx1-lacZ KI (v-viii) embryos. (i,v) LacZ expression in whole mounts. White arrowheads point at the area of the BI; black arrowheads point at the anterior paired dorsal aortae, outside of the hematopoietic PAS region. (ii,vi) LacZ expression in YS BI. (iii,vii) Expression in posterior paired dorsal aorta in the PAS region. Arrowheads indicate expression in the wall of the dorsal aortae. (iv and viii) Enlargement of boxed areas in panels iii and vii, respectively. Photographs in panels i and v were made using a 1.6× objective lens; panels ii-iv and vi-viii: 20× Nomarski objective. (C) LacZ expression in E10 hsp68LacZ + 23 F0 transgenic (i-iv) and Runx1-lacZ KI (v-viii) embryos. (i,v) Expression in the aorta in the AGM region (dorsal = up). Arrowhead in panel Ci indicates + 23 enhancer activity in a cluster of hematopoietic cells. LacZ expression is also seen in few individual cells in the endothelial layer and the mesenchyme (arrows). LacZ expression in the Runx1-LacZ KI (v) also includes a wider population of endothelial and mesenchymal cells. (ii,vi) Expression in hematopoietic cell clusters of the umbilical artery (arrowheads) and cells in the endothelial wall (arrows). (iii,vii) Expression in YS blood cells. (iv,viii) LacZ expression in FL. Black arrowheads indicate expression in individual hematopoietic cells. The white arrowhead points at the FL capsule. Original magnification, ×200.