GC11E10-lacZ expression. Overnight X-gal staining showed fusion transcript expression at medium to high levels in most undifferentiated ES cells (A). In attached EB cultures, lacZwas expressed within blood islands (bi) and the associated vascular endothelium (arrows; B). Differentiation of clone GC11E10 on OP9 stromal cells demonstrated lacZ expression in mesodermal (M) colonies (C) and a proportion of hematopoietic clusters long arrow as well as all large hematopoietic cells (short arrows; D). Overnight whole mount X-gal staining of an e9.5 chimeric embryo and yolk sac demonstrated lacZ expression in the dorsal aorta, heart, liver, and vasculature (E). LacZ expression in the yolk sac was confined to endothelial and hematopoietic cells (F and G). LacZwas expressed by the endocardium and circulating blood cells in the heart (H) and by the intersomitic endothelial cells (arrow; I).