The MMTV/neu transgenic (Tg) mice spontaneously develop mammary tumors stochastically after a long latent period, suggesting that the c-neu/erbB2 oncogene is not sufficient for tumor formation. To identify putative collaborator (s) of the c-neu/erbB2, we used the provirus insertional mutagenesis approach with mammary tumors arising in MMTV/neu Tg mice infected with the mouse mammary tumor virus (MMTV). The Notch1 gene was identified as a novel target for MMTV provirus insertional activation. In Notch1-rearranged tumors, the Notch1 gene was interrupted by the MMTV provirus insertion upstream of the exons coding for the TM domain. These insertions led to overexpression of novel 5′ truncated∼ 7 kb RNA coding for 280 kDa mutant protein harboring only the Notch1 ectodomain, N (EC) mut. These may be involved in tumor formation. Another consequence of these insertions was the expression of truncated 3′ Notch1 transcripts (3.5–4.5 kb) and proteins (86–110 kDa) deleted of most of the extracellular sequences (Notch1 intra). We found that 3′ truncated Notch1 intra can transform HC11 mouse mammary epithelial cells in vitro. Deletion analysis revealed that the ankyrin-repeats and the domain 1 (aa 1751–1821) are required, while a signal peptide, the two conserved cysteines (C 1652 and C 1685) and the OPA and PEST sequences are dispensable for transformation. These results indicate that the N-terminally truncated Notch1 intra protein behaves as an oncogene in this system.