Abstract Breast cancer (BC) is the most common cancer among women. Viruses have long been suspected as a cause of breast cancer. However, despite the identification of virions in human BC cells and human milk and numerous studies that suggest possible connection between breast cancer and Epstein-Barr virus (EBV), mouse mammary tumor virus (MMTV), and human papilloma virus (HPV), the tumorigenic potential of these viruses has not been proven in BC. Our preliminary data offer the compelling evidence that a filterable transforming agent is present in the supernatant of a BC cell line that we established in vitro. This agent is transmissible, transforms primary human mammary epithelial cells in vitro, and confers these cells the tumorigenic phenotype in vivo. Based on preliminary results, we hypothesize that a virus accounts for the observed human mammary epithelial cells transforming activity. This application has the very focused objective of isolating and characterizing this transforming agent employing comprehensive and complementary proteomics and transcriptomics approaches and in vitro genetic validation systems. The results of this study will provide the first evidence of a human mammary epithelial cells transforming agent and shed new light into tumor pathogenesis. Importantly, the isolation of a BC transforming virus will allow the development of novel diagnostic tools, vaccines, and specific inhibitors of significant therapeutic value.