Finally, we report that deletion of Esco2 to kill proliferating alveolar fibroblast progeny or deletion of Ect2 to inhibit cytokinesis in the same cells both caused a reduction in fibroblast expansion and led to protection from PF and the resultant hypoxemia caused by either bleomycin or silica, suggesting that fibroblast proliferation is functionally important for driving fibrosis. This evidence concerns the gene ESCO2 and pemphigus foliaceus.