ESR1 and breast cancer: To study the role of Ret in ER+ breast cancer, we focused on four models: human ER+ T47D cells, MCF7 cells and their aromatase-expressing derivative (MCF7/Aro) (Boulay et al, 2005), which respond to the estradiol (E2) precursor androstenedione (Δ4A); and the mouse J110 cell line.