This animation visualises research published in Nature Medicine (Vol 15, Issue 8, 2009) by the laboratory of Jane Visvader and Geoffrey Lindeman. The mammary gland is comprised of three main cell types; alveolar, ductal and myoepithelial cells. Stem cells can develop into any of the three cell types through a series of intermediate cell stages. One intermediate is the luminal progenitor cell, which develops into either alveolar or ductal cells. The paper describes how an aberrant form of a luminal progenitor cell is involved in the development of some forms of breast cancer.