In mouse mammary tumor virus (MMTV) infected mice, three identifiable stages of mammary tumorigenesis can be biologically defined: preneoplastic hyperplastic nodules, malignant tumor, and distant metastatic lesions (primarily in the lung). MMTV is a biological carcinogen which induces somatic mutations as consequence of its integration into the host cellular genome. Each stage of mammary tumorigenesis appears to result from the clonal outgrowth of cells containing additional integrated proviral MMTV genomes. This phenomenon has provided the basis for an approach to identify genes which, when affected, may contribute to progression through the different stages of mammary tumorigenesis. Eight different genes (Wnt1, Wnt3, Wnt10b, Fgf3, Fgf4, Fgf8, Int3, and Int6) have been shown to be genetically altered in multiple mammary tumors as a consequence of MMTV integration. Although the significance of the human homologs of these genes as targets for somatic mutation during human breast carcinogenesis is only now being explored, it is clear that this work has led to a new appreciation of the complexity of the genetic circuitry that is involved in the control of normal mammary gland growth and development. It seems likely that some of the mutations induced by MMTV, and the signaling pathways in which these target genes take part, will be relevant to the progression from preneoplastic lesions to distant metastasis in human breast cancer.