What exactly is breast cancer? what causes it?

Malignancy of breast: Breast cancer is a malignancy that most often develops in the duct cells of breast tissue. Less frequently it appears from lobules at the terminal duct responsible for milk production. The most common cause is environmental with a virus such as MMTV entering the Wnt-1 gene of the duct cell to produce B catenin which helps convert the cell, possibly with EBV to the malignant phenotype.