The INT3 gene is frequently rearranged in mouse mammary tumor virus (MMTV)-induced mammary tumors of the CzechII mouse strain. We have completed the nucleotide sequence of the normal 6.5 Kb INT3 RNA and defined the intron/exon boundaries of the gene. The open reading frame of INT3 RNA should encode a 200 kd protein which shares 60% homology with the mouse homologue of Drosophila NOTCH. INT3 is unique among other members of the NOTCH family by containing 29 instead of 36 EGF-like repeats in the extracellular domain of the gene product. Five novel EGF-like repeats have been created as consequence of apparent small deletions which have occurred within the coding region for the extracellular domain during evolution. Nucleotide sequence analysis of host-viral junction fragments from nine independent MMTV-induced mammary tumors containing a rearranged INT3 gene reveals that all of the integration events occur within a 174 bp region 3' of the sequences encoding the LIN12 repeats in the INT3 extracellular domain and 5' of the sequences encoding the transmembrane domain. Therefore, the only tumorigenic INT3 mutations resulting from MMTV proviral insertions are those which results in the expression of the intracellular domain. This strongly suggests that MMTV-induced activation of INT3 is manifest in the absence of the regulatory action of the extracellular domain, including the LIN12 repeat sequences, leaving the expressed intracellular domain constitutively free to function in its role in mammary tumorigenesis.