Although homozygous Apc (ApcΔ580/Δ580) mutations induced either by K14 or WAP-promoters invariably results in squamous metaplasia, Cre-mediated Apc heterozygosity (ApcΔ580/+) in K14-cre; ApcCKO/+ mice developed mammary tumors with high penetrance. The gene discussed is APC; the disease is breast cancer.