Mammary carcinomas in certain mice strains are induced following infection by the MMTV. Insertion of MMTV provirus into the int-1 and int-2 loci results in transcriptional activation of these two proto-oncogenes and is thought to be a key step in breast tumorigenesis in mice. A viral etiology for human breast cancers, though proposed several years ago, is far from proven. However, morphological structures resembling Bittner's particles have been observed earlier in about 39% samples of breast tumor and milk sediments of Parsi women. We therefore investigated in the Parsis, the structural integrity of two potential sites of integration (int-1 and int-2) of the hypothetical human mammary tumor virus and have used for this purpose a large number of patient DNA. The results obtained however failed to distinguish any major structural change existing at the int-1 or the int-2 sites that may point to a proviral integration event having occured in human breast cancers. We have, however, observed differences in the physical structure of the int-1 map, compared to the one reported and sequenced, and have therefore felt it necessary to present a new one indicating our findings, notably, the polymorphic PvuII sites. In addition, we report a single case of Parsi woman, with infiltrating grade 3 ductal carcinoma, whose DNA contains an alteration in the int-2 structure.

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