K17G2-lacZ expression in vitro and in vivo. Overnight X-gal staining showed fusion transcript expression at medium intensity in most undifferentiated K17G2 ES cells (A). The fusion transcript was expressed in the blood island (bi) and some of the associated vascular endothelium (arrows) in attached EB culture (B). Differentiation of clone K17G2 on OP9 stromal cells demonstrated lacZ expression in mesodermal (M) colonies (C) and hematopoietic clusters (arrow; D). X-gal staining of an e10.5 F₁ embryo demonstrated limitedlacZ expression in the embryo (whole mount; E). An X-gal stained e12.5 F₁ embryo demonstrated lacZexpression in the pericardium (F) and vascular endothelium and circulating hematopoietic cells (G). In addition, the myocardium (H) and the dorsal root ganglia (I) also express the lacZ fusion protein.