Figure 2.

Model of stem/progenitor cellular hierarchy in the mouse mammary gland. The pluripotenet stem cell gives rise to duct-limited and lobule-limited (PI-MEC) progenitors. Both progenitor populations are multipotent. The lobule progenitor gives rise to ER^+/− and PR^+/− luminal epithelial cells and myoepithelial cells. Duct-limited progenitors also give rise to ER^+/− and PR^+/− luminal epithelial cells as well as the cap cells during ductal elongation. The cap cells ultimately differentiate into the myoepithelial cells of the ducts. Arrows indicate potential and identified targets of the oncogenic stimuli listed.