Histologic analysis of single enhancer constructs reveals multiple elements conferring weak hematopoietic expression. Histologic sections of FL, dorsal aorta (DA), heart (H), yolk sac (YS), and peripheral vessels (V) from transgenic mouse embryos at E12.5. X-Gal reporter expression driven by the LMO2 349-bp minimal pP (pPLacZ) combined with each of the 8 enhancers (−90, −75, −70, −64, −25, −12, +1, +7). As previously shown, the pPLacZ construct can mediate only a weak endothelial-specific activity (Figure 3B). The endothelial expression is enhanced by collaboration of pP with any of the presumed enhancer elements under investigation. The strongest endothelial enhancer activity is conferred by the elements −90 and +7, which are able to direct expression to the DA and endocardium. Most interestingly, the elements −90, −75, −64, −25, −12, and +1 confer weak and focal FL expression, whereas the element −75 mediates weak staining to circulating erythrocytes (see inset in FL, H, and V). However, overall hematopoietic staining is still less pronounced compared with the Lmo2 LacZ KI (Figure 3A).