The erbB2/neu gene is known to encode a tyrosine kinase transmembrane receptor of the HER family and is frequently amplified and overexpressed in human breast cancer. In these cancers other genetic modifications, such as gene amplification, deletion or mutation, are likely to contribute to the development of malignant tumors. Loss of heterozygosity (LOH) is the most common type of mutation seen in human primary breast carcinoma. To study the molecular events involved in erbB2/neu transformation several groups have studied transgenic models with erbB2/neu under control of the mouse mammary tumor virus long terminal repeat (MMTV-LTR). These mice develop mammary tumors stochastically and after a long latency peroid.