Lung development in the mouse begins on embryonic day 9.5 (E9.5) when the lung primordium appears as a ventral bud in the primitive foregut [4] . Airway branching begins around E9. [5] [6] [7] [8] [9] [10] [11] [12] and continues through the ''pseudoglandular'' [E12-E16.5] and ''canalicular'' [E16.5-E17.5] stages. Thereafter, during the ''saccular'' stage [E17.5 to postnatal day 4 (P4)] the distal airways form the saccular units which are further subdivided by secondary septae formed during the alveolar stage (P4-P28 in mice) to form mature alveoli. This sequence of events is tightly controlled by the concerted action of growth factors, transcription factors, and mechanical forces [5, 6, 7] .