Approximately 40% of human breast cancers contain gene sequences that are remarkably similar to a retrovirus known to cause breast cancer in domestic mice.  Furthermore, the highest incidence of human breast cancer worldwide occurs in geographic locations where the domestic mouse is native or introduced to the area. 

The search for a virus related to human breast cancer may be as close as the mice that share the world in which we live.  Once a virus enters a human breast cell it can only replicate if the cell in which it grows is actively growing and dividing.  Human breast cells grow and divide in response to circulating estrogen, both endogenous (produced in our bodies by our ovaries, adrenal glands, and fat tissues) and exogenous (ingested as birth control pills, hormone replacement therapy, or environmental pollutants).  When a breast cell that contains a viral sequence divides, it releases more viral particles that can infect other breast cells…and the cycle continues.  The possibility of a ubiquitous breast cancer-producing virus, replicating in the presence of estrogenic stimulation, may be a lethal combination, one that merits further investigation.

If the mouse mammary tumor virus can be shown to cause 40% of human breast cancer, and if a vaccine can be made that provides immunity to this virus then a substantial gain can be made in the effort toward primary prevention of breast cancer.

The following articles are referenced for your review.  Although these are scientific papers, published in peer-reviewed journals, and may seem to be overly technical in part, they are understandable and we think you will find them very interesting.

• Possibilities of a viral etiology for human breast cancer.
  Pogo B, Biological Trace Element Research
  Vol. 56, 1997, pp 131-142
  
  
• Identification of human homologues of the mouse mammary tumor virus receptor. 
  Stewart A, Archives of Virology,
  Vol. 147, 2002, pp 577-581
  
  
• Breast cancer incidence highest in the range of one species of house mouse, Mus domesticus. 
  Stewart THM, British Journal of Cancer,
  Vol. 82, 2000, pp 446-451

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