Myoepithelial differentiation is 'constitutive' to specific tumor types, for example adenosquamous carcinomas, adenomyoepitheliomas, and adenocarcinomas associated with the active mouse mammary tumor virus (MMTV) and the wingless-related MMTV integration site 1 (Wnt1) transgene. The gene discussed is WNT1; the disease is adenosquamous carcinoma.