From E16 onwards, expression of eGFP was detected in both proximal and distal epithelial compartments.In the proximal epithelium (i.e. the developing airways) eGFP expression exhibited a proximal-distal axis pattern, with high eGFP expression in bronchi and large airways and surprisingly no expression at the distal portion of the growing airway tree. Interestingly, isolated clusters of eGFP pos cells were observed at the tips of airway branch-points. In the distal lung compartment, eGFP expression was detected in cuboidal epithelial cells of the developing saccules ( Figure 1B ). As lung morphogenesis approached completion, the eGFP signal was reduced and in adult mice,(,2 months old), few eGFP pos cells were detected scattered in the epithelium of bronchi and large airways, in the alveolar compartment and in a subpopulation of cardiac cells in the outer layer of the tunica media of the pulmonary veins ( Figure 1B ) [40] .