Additionally, blocking ERa decreased H19 expression in the luminal progenitors and led to the development of smaller luminal colonies. We further showed that knocking down the H19 gene in the luminal progenitors significantly decreased the colony-forming potential of the luminal progenitors, and this phenotype could not be rescued by the addition of estrogen. Lastly, we explored the clinical relevance of the estrogen-H19 signaling axis in breast tumors and found that ERa(+) tumors exhibited a higher expression of H19 as compared with ERa(-) tumors and that H19 expression showed a positive correlation with ERa expression in those tumors.