Around E11, eGFP expression was confined to the endothelial cells of the large pulmonary vessels, and the developing vascular network in the parenchyma. A very small fraction of sub-epithelial smooth muscle cells (SMCs) expressed very low levels of eGFP at E12 ( Figure S1 ). However, from E13.5 onwards robust eGFP expression was detected in sub-epithelial SMCs located at the proximal regions of the developing airways (Figures 1 and Figures  S1 and S2) .