Hypothetical scheme on the effects of β-catenin signaling activation on stem/progenitor population and tumor induction. A. During normal homeostasis, basal and luminal progenitors with limited self-renewal potential, maintain their respective lineages. B. Constitutive activation of β-catenin signaling leads to enhanced basal progenitor self-renewal and expansion preventing myoepithelial differentiation and giving rise to basal-like tumors. Both, basal progenitor amplification and tumorigenesis are Myc-dependent.