Min (multiple intestinal neoplasia) is a mutant allele of the murine Apc (adenomatous polyposis coli) locus, encoding a nonsense mutation at codon 850. Like humans with germline mutations in APC, Min/+ mice are predisposed to intestinal adenoma formation. The number of adenomas is influenced by modifier loci carried by different inbred strains. One modifier locus, Mom-1 (modifier of Min-1), maps to distal chromosome 4. Intestinal tumours from both B6 (C57BL/6J) and hybrid Min/+ mice show extensive loss of the wild-type allele at Apc. B6 Min/+ female mice are predisposed to spontaneous mammary tumours. The incidence of both intestinal and mammary tumours can be increased in an age-specific manner by treatment with ethylnitrosourea (ENU). Min mice provide a good animal model for studying the role of Apc and interacting genes in the initiation and progression of intestinal and mammary tumorigenesis.