To further compare the proliferative potential of the eGFP pos and eGFP neg cells different epithelial populations were isolated from dissociated lung tissue by cell sorting and analyzedin-vitro for colony-forming ability. Following a previously described protocol [38] , EpCAM pos , Lineage negative (i.e. CD45 neg -CD31 neg -CD34 neg ) cells from BRE-eGFP transgenic animals were subdivided into Sca1 neg and Sca1 low cells. Finally the Sca1 neg and Sca1 low cells were separated by sorting into eGFP neg , eGFP low and eGFP high cells respectively ( Figure 7D ). Isolated cells were cocultured with MLg feeder cells in a 3-dimensional Matrigel culture system [38] with a modification developed by some of us (HC and BRS) that involved co-culture of the cells in Matrigel for eight days in the presence of the TGFb receptor inhibitor SB431542 (10 mM), followed by an additional four days in the absence of inhibitor.