Strong eGFP expression was detected in the developingsubepithelial SMC layer of the proximal airways from E14.5 to P1 ( Figure 3A and Figures S1 & S2) . The eGFP expression, which interestingly involved only a portion of the sub-epithelial SMCs, was undetectable by P15. It has been proposed that sub-epithelial SMCs originate from a pool of sub-mesothelial, FGF10-expressing, mesenchymal cells. Under the influence of several signalling pathways, such as those mediated by FGF, BMP, Shh, and Wnt, these cells proliferate and passively translocate to surround the proximal portion of the developing airway tree [35, 43, 44, 45, 46] . http://www.sciencedirect.com/science/article/pii/S0012160611 010189 -bb0125To map more precisely the temporal activation of the BRE-eGFP reporter during development of the subepithelial SMC compartment we isolated P11, P12 and P13.5 lungs from the BRE-eGFP reporter animals and analysed coexpression of eGFP and SM22a, a marker of both primitive and mature SMCs [47] , or aSMA a marker of more differentiated SMCs. As shown in Figure S1B , SM22a expression was observed in a sub-epithelial population of cells around the developing proximal airways already at embryonic stage E11, aSMA expression was evident only in the most proximal portion of the SM22a pos zone and eGFP expression was clearly detected in the aSMA pos zone only after embryonic stage E13.5 (very few aSMA pos cells expressed very low levels of eGFP at E12). These findings are consistent with the notion that canonical BMP signalling does not involve primitive SMCs but rather mature aSMA expressing sub-epithelial SMCs.