The localization of eGFP pos cells in the vicinity of NEBs and the minimal overlap between cells expressing eGFP and markers for specialized secretory (CC10) or ciliated cells (FoxJ1) prompted us to investigate whether active canonical BMP signaling may define cells with an immature progenitor phenotype. Thus, a single two hour pulse of BrdU was given to pregnant mothers at E17 to label cycling epithelial cells which were subsequently detected by staining with anti-BrdU antibodies. BrdU pos cells accounted for ,6.7% of the surface airway epithelium in conducting airways and were uniformly distributed throughout the airway tree ( Figure 7A ). This pattern is compatible with the findings of Giangreco et al. [56] and Rawlins et al. [57] demonstrating that randomly distributed progenitor cells contribute to normal epithelial homeostasis. Of the BrdU pos cells ,77.8% were eGFP neg and ,22.2% were eGFP pos , indicating that both populations contribute to the expansion of the airway epithelial compartment ( Figure 7B ). Despite the dominance of the eGFP neg -BrdU pos cells (in absolute numbers), ,13.4% of the eGFP pos were BrdU pos, compared to ,5.5% for the eGFP neg cells ( Figure 7C ).