Six1 transcription factor is critical for coordination of epithelial, mesenchymal and vascular morphogenesis in the mammalian lung

► Six1 is expressed in lung distal epithelium and mesenchyme. ► Six1−/− lungs are hypoplastic with reduced branching. ►Six1 controls both epithelial and mesenchymal cell proliferation. ► Six1 induces epithelial progenitor cell proliferation but inhibits differentiation. ► Six1 coordinates Shh-Fgf10 signaling during lung development.