For the past 20 years the mouse has served as a workhorse for studying the molecular underpinnings of human breast cancer. While some genetically engineered mouse mammary tumor models do not accurately recapitulate human disease (that is, the MMTV-Neu model and HER2-overexpressing human cancers), additional tumor models exist for studying the initiation, progression, and treatment of human breast cancer. The relationships between normal mouse mammary epithelial cells and tumor cells, however, have remained poorly defined. A recent article in Breast Cancer Research has shed light on these relationships.