Mouse models of breast cancer have been used as models of human breast cancer since the 1890s (1). The landmark description of the histopathology of “spontaneous” mouse mammary cancer was written in 1906 (2) and reviewed in the English literature in 1911 (3). Coupled with Jensen’s transplantable mammary cancer line, these papers were the foundations of experimental cancer research (4). The experimental era leading up to modern times used the mouse to establish the role of hormones, transplantation biology, and neoplastic progression (1). However, “spontaneous” mammary cancer in mice proved to be virus induced and initiated by insertion activation of developmental genes not directly related to human breast cancer, and the tumors were not morphologically similar to human.