The study of experimentally induced mammary tumors has focused primarily on several mouse strains that are infected with the mouse mammary tumor virus (MMTV). MMTV appears to induce tumors by acting as an insertional mutagen that leads to the activation of a previously silent cellular gene or the rearrangement of a normally expressed gene (int genes). We have found a dichotomy in the frequency with which the wnt-1 gene is activated in tumors arising within preneoplastic hyperplastic outgrowth lines (6%) and those arising in situ (52%) in the mammary glands of C3H breeders. It would appear that wnt-1 activation provides a proliferative advantage to transformed mammary epithelial cells in intact C3H mammary glands.We have determined the nucleotide sequence of the 2.3kb RNA species whose expression is activated by MMTV insertion in the int-3 locus in mammary tumors. It encodes a 57kD protein which is 50% identical to the intracellular portion of the neurogenic Drosophila notch gene product. A common characteristic of these proteins is six nearly contiguous 32 amino acid repeats which are bounded by the PEST amino acid sequence motif that is characteristic of proteins having a rapid turnover. We have used the "normal" HC11 mouse mammary epithelial cell line to study the biological activity of the int-2 and int-3 gene products. Int-2 is a member of the fibroblast growth factor (FGF) gene family. Activation of expression of either int-2 or int-3 in HC11 cells induces anchorage-independent growth in soft agar. Moreover, the autocrine expression of int-2 in HC11 cells abrogates their requirement for either epidermal growth factor of bFGF priming prior to induction of beta-casein expression with lactogenic hormones. A transgenic mouse strain has been established containing activated int-3 as the transgene. Focal and often multiple poorly differentiated mammary and salivary gland adenocarcinomas occur in 100% of the transgenic mice between 2 and 7 months of age. Significantly, mammary glands were arrested in development and were lactation deficient in all female int-3 mice. In other studies, we have developed a novel approach to introducing genes into primary mammary epithelial cells to test their biological activity in mammary fat pads, using a retroviral shuttle vector containing LacZ reporter gene.