ESR1 and adenocarcinoma: In conclusion, we have shown that by altering the balance of EGFR familysignaling in favor of ERBB3/ERBB4 we can prevent squamous metaplasia of mammaryepithelial cells in vitro, that human cells withmultiple defined oncogenic changes can replace the luminal cell layer in the mousemammary gland with a morphologically normal luminal layer of human cells, and thataddition of a single activated oncogene, mutant PIK3CA, is sufficient to convertthese cells into invasive ERα + adenocarcinoma cells.