On the contrary to other, more common gene mutations leading to breast cancer (e.g. ErbB2) which are likely to affect more committed progenitor cells, Wnt/β-catenin signaling activation presumably affects self-renewal and differentiation capacity of the mammary primordial stem cell or in very early progenitors as shown by the metaplastic changes and the presence of both myoepithelial and luminal lineages in Wnt-driven mammary tumors. The gene discussed is ERBB2; the disease is breast carcinoma.