Fig. 8. Schematic of the proposed interactions between PR and ß-catenin signaling. In PR^-/- glands, cells at ductal tips are intrinsically responsive to ß-catenin signaling (blue). In PR^+/- glands, PR induces competence to respond to ß-catenin signaling within a subset of alveolar progenitors along lateral borders (blue) and possibly designates other bipotent (gray) and ductal (yellow) progenitor populations. ß-catenin induces alveologenesis at ductal tips of PR^-/- mice and at tips and along lateral borders of PR^+/- mice. It also triggers the secretory differentiation program, resulting in distended secretion-filled lumen (pink) in PR^-/- mice. Secretory differentiation is restrained by PR. In the normal virgin mammary gland, PR induces a non-uniform PR expression pattern and competence to respond to ß-catenin in alveolar progenitors (blue) during ductal maturation. During early pregnancy, PR-WNT4 signaling induces expansion of ductal progenitors (yellow) to form side-branches through ß-catenin-independent routes. PR and PRL cooperate to induce alveologenesis, a process that is ß-catenin-dependent. Later in pregnancy, PR restrains and PRL promotes alveolar differentiation. ß-catenin is required for alveologenesis and may participate at multiple steps in the secretory differentiation pathway.