Dynamic expression profile of Tbx4-rtTA-mediated Tet-On targeting system shown by the triple transgenic reporter (Tbx4-rtTA/TetO-Cre/mT-mG). (A) Tbx4-rtTA-mediated Tet-On system targeted lung mesenchyme was initiated immediately after lung morphogenesis at E9.5. Dox induction was given at different time windows of early gestation, and the sagittal frozen sections of E13.5 embryos were used to detect mGFP (green) and mTomato (red) expression. Nuclei were counterstained with DAPI (blue). Lung tissue (L) was marked on the left side of the dotted line, and vertebrae (V) were located on the right side. (B) Efficiencies of cell targeting by the Tbx4-rtTA mediated Tet-On inducible system at different prenatal and postnatal stages. The time windows of Dox induction are indicated above each panel, and the ages of the examined lung specimens are specified inside each panel. DAPI, 4',6-diamidino-2-phenylindole; Dox, doxycycline; E, embryonic day.