ABSTRACT Alveologenesis occurs during postnatal development in humans and mice and this process allows for the growth of gas exchange surface area of the lung. One of the key events during this phase is the establishment of the elastin-based matrix in the distal airway. The incorporation of elastin into the existing lung matrix provide the elasticity that allows the distal airways to stretch and recoil effectively during breathing and mesodermally-derived SMA+ lung fibroblasts are key drivers of this process. Our previously published work has shown that the Hox5 genes are exclusively expressed in the mesenchyme of the lung and that loss of all three Hox5 genes leads to early, severe developmental lung defects and neonatal death. Four-allele, compound Hox5 mutant mice (Hox5 AabbCc) are born in Mendelian ratios and are phenotypically normal at birth, however, they develop alveolar simplification at postnatal stages. Consistent with a direct role for Hox5 genes in alveologenesis, the expression levels of all three Hox5 genes are highest during early postnatal stages when the bulk of alveologenesis occurs, higher than observed at any embryonic stage and these genes remain expressed through adult life. Using a newly generated conditional allele for Hoxa5, we show that conditional deletion of Hoxa5 in the lung mesenchyme beginning at birth results in an alveolar simplification phenotype postnatally. Hox5 mutant animals exhibit abnormal myofibroblast morphology and impaired function. Hox5 mutant fibroblasts demonstrate defects in cell adhesion and the expression of Integrin 5 and 1 are down- regulated. The continuing importance of Hox5 function at all stages is highlighted by surprising preliminary evidence that deletion of Hoxa5 at later stages (after the establishment of the elastin-based matrix) leads to rapid loss of the integrity of the elastin matrix. In this proposal, we will interrogate the cellular and molecular mechanisms of Hox5 regulation of lung mesenchyme during alveolar development, remodeling and homeostasis.