Tg(Alk1-cre)-L1 mice express the Cre recombinase predominantly in the pulmonary vascular endothelial cells. The cells in which Cre–mediated recombination has occurred were visualized by staining the Tg(Alk1-cre);R26R bigenic embryos with X-gal for the β-gal activity at E10.5 (A-D), E13.5 (E-H), and E15.5 (I-K) stages. (A,B) X-gal-positive staining is visible in the blood vessels throughout embryos in the Tg(Alk1-cre)-B (A) and -D (B) lines. (C) Transverse sections of X-gal stained Tg(Alk1-cre)-D embryo showing lacZ expressions in the vascular ECs, endocardial cells in atria and ventricles, and mesenchymal cells in the atrioventricular cushion (AVC). (D) In contrast with that in the B and D lines, almost no lacZ expression was detected in the L1cre embryos; only a spotty staining pattern in the head region (arrow). (E,F) Dorsal aorta (DA) view of the heart and lungs of L1cre:R26R embryos stained with X-gal, showing a strong lacZ expression in the lung in comparison with a patch staining in the heart (E) and body trunk (F). (G,H) Histologic sections of the X-gal stained lung was counterstained with NFR (G) or costained with anti-PECAM antibodies (H). The inset in panel H is a magnified view of the area indicated by the arrow. Note that X-gal-positive cells resided in pulmonary ECs, but only in a subpopulation of PECAM-positive cells. (I) Ventral view of the X-gal stained L1cre:R26R lung attached to the body trunk. The heart was removed for clarity of the view. Note a strong X-gal staining in the lung but not in the body trunk. (J,K) Histologic sections demonstrate that most PECAM-positive cells are positive for X-gal staining in E15.5 embryonic lungs, yet no X-gal-positive cells were detected in airway epithelial and smooth muscle cells. Insets are magnified views of the areas indicated by the arrow in each panel.