The relevance of Wnt4 is confirmed by the phenotype of conditional knockouts of mammary Wnt4 expression in mice that have impaired ductal side branching, while overexpression of stabilized β-catenin in luminal epithelium results in precocious lobulo-alveolar development, alveologenesis, and neoplasia (112). The gene discussed is WNT4; the disease is neoplasm.