Previous studies have reported the cloning and identification of the int-5, a novel mouse mammary tumor virus (MMTV) integration locus involved in mammary tumorigenesis. Here we report the characterization of the 5.5 kb region from this novel MMTV integration site. Our results show that the region after the MMTV integration site, a 258 bp sequence is homologous (100%) to the mouse early transposon (mETn) long-terminal repeat and other sequences of the this transposon are not present in the 5.5 kb region. Our results also show for the first time the tumor specific expression of mETn and expression of the region downstream of the MMTV integration site that is homologous to mETn-LTR in D2 mammary tumors.