Mouse mammary tumorigenesis as a result of mouse mammary tumor virus (MMTV) integrations has helped to identify a wide variety of interesting genes that play a role in mammary development and tumorigenesis. Several such genes int1/wnt1, wnt3, wnt 10B, int2/fgf3, fgf4, int3/notch and int6 have been shown to be genetically altered in naturally formed mammary tumors as a consequence of MMTV integration. Some of these genes have been well characterised and examined in in vivo breast cancer transgenic models for their potential for tumorigenesis. Overexpression of one or more of these genes have resulted in a striking proliferation of mammary gland epithelium of both female and male transgenic mice. Our own studies have demonstrated overexpression of int5/aromatase in mammary glands of virgin and postlactational females leads to the induction of various preneoplastic and neoplastic changes that are similar to early breast cancer, that may, in turn, increase the risks for developing breast cancer. Therefore, further understanding of these genes should provide new insights to their involvement and mechanism of action in breast cancer.