Description
Mammary alveologenesis is abrogated in the absence of the transcription factors STAT5A/5B that mediate cytokine signaling.  To reveal the underlying causes for this developmental block we studied mammary stem and progenitor cells.  While loss of STAT5A/5B did not affect the stem cell population and their ability to form mammary ducts, luminal progenitors were greatly reduced and unable to form alveoli during pregnancy.  Temporally-controlled expression of transgenic STAT5A in mammary epithelium lacking STAT5A/5B restored the luminal progenitor population and rescued alveologenesis in a reversible fashion in vivo.  Taken together, STAT5A is necessary and sufficient for the establishment of luminal progenitor cells.