Differential targeting of lung smooth muscle cells by the Tbx4 lung enhancer at different gestational ages. (A) Tbx4 lung enhancer-mediated Tet-On induction activity was only detected in cytokeratin (Cyt)-negative cells, shown by GFP and Cyt co-immunostaining. In addition, not all GFP-positive cells were lacZ-positive, shown by GFP and LacZ co-immunostaining. The lung specimens were taken from the E18.5 triple transgenic fetuses with Dox induction time windows indicated above the panels. (B-D) Co-immunostaining of GFP, LacZ and SMA for the lungs of E18.5 (B), E15.5 (C), and E18.5 (D) triple transgenic fetuses with different Dox inductions indicated above the panels. a: airway; v: vasculature. Dox, doxycycline; SMA, α-smooth muscle actin.