Differential targeting of lung vascular endothelial cells by the Tbx4 lung enhancer at different gestational ages. (A) In E18.5 lung of the triple transgenic mice with Dox induction from E6.5, PECAM-1-positive endothelial cells were GFP positive, but LacZ negative. (B-C) in the E15.5 lung of the triple transgenic mice with different Dox induction time windows as indicated above the panel, vascular endothelial cells were GFP-positive if Dox induction was given prior to E10.5 (B), but LacZ expression was already negative. Consistently, Dox induction from E11.5 to E15.5 was not able to target vascular endothelial cells, shown by double negative staining for GFP and LacZ (C). V: vasculature. Dox, doxycycline; E, embryonic day; PECAM-1, platelet endothelial cell adhesion molecule.