In conjunction with our studies on mammary tumorigenesis in feral and inbred mice, our efforts have focused upon understanding the cellular basis of malignant progression in the mammary gland. Taking the point of view that mammary carcinomas arise as clonal populations of transformed tissue-specific stem cells and their differentiating progeny, we have initiated a long-term project aimed at elucidating the cellular, molecular, and genetic events underlying mammary epithelial cell growth, regeneration, and functional development.[unreadable] [unreadable] The reproductive capacity of the mammary epithelial stem cell is reduced coincident with the number of symmetric divisions it must perform. In a study of FVB/N mice with the transgene, WAP-TGFb1, we discovered that mammary epithelial stem cells were prematurely aged due to ectopic expression of TGF-b1. To test whether premature aging of mammary epithelial stem cells would have an impact on susceptibility or resistance to mammary cancer, female littermates from FVB/N X WAP-TGF-b1 mating were injected with mouse mammary tumor virus (MMTV) at 8-10 weeks of age. A total of 44 females were inoculated, maintained as breeders and observed for tumor development for up to 18 months. Only one mammary tumor appeared in 17 TGF-b1 females while 15 were collected from 29 wild type sisters. Premalignant mammary epithelial cells in infected glands were identified by transplantation of single cell (1x105) suspensions into nulliparous hosts and testing for hyperplastic outgrowth. Although the number of positive takes was significantly reduced with TGF-b1 cells, both MMTV-infected TGF-b1 and wild type cells produced hyperplastic outgrowths suggesting that premalignant transformation was achieved in each group. The results suggest a positive correlation between the procreative life-span of mammary epithelial stem cells and mammary cancer risk.[unreadable]