Much of our knowledge about the intricate pathways and molecular mechanisms involved in the conversion of a normal mammary epithelial cell to malignancy derives from studies on mammary tumorigenesis induced by the retrovirus mouse mammary tumor virus. In addition, three DNA tumor viruses, simian virus 40, polyomavirus, and human papillomavirus, have been instrumental in dissecting the series of steps comprising neoplastic progression of mammary epithelium, particularly with cultured human breast cells. Endogenous transposons are analogous bioagents receiving increased attention recently. At least 10% of the cell genome consists of transposable elements, a growing number of which have been implicated in mutagenizing DNA in a variety of human tissues and disorders. Research efforts have therefore intensified to determine if endogenous elements such as retrotransposons participate in the development of breast cancer in animals and humans.