Epimorphin/syntaxin-2 (EPM) is a plasma membrane–anchored protein that has at least two distinct functions depending on its membrane topology: vesicle fusion when localized to the cytoplasmic surface and morphogenic signaling when localized to the extracellular surface. Transgenic mice that express full-length extracellular EPM fused to the NH₂-terminal signal sequence of interleukin-2, under the control of the whey acidic protein (WAP) gene promoter, exhibit aberrant mammary gland morphogenesis associated with increased expression of CCAAT enhancer binding protein β (C/EBPβ). Here we report that aged nulliparous and uniparous female WAP-EPM transgenic mice develop alveolar hyperplasias and well-differentiated adenocarcinomas that express high levels of C/EBPβ, keratin-14, matrix metalloproteinase-3, and β-catenin. This study reveals another pathway in which overexpression and alteration of a normal morphogenic process promote the development of cancer in the mammary gland.