Breast cancer is the most common type of tumor found in women. Each year in Germany alone, 69,000 new cases are diagnosed. Around 80 percent of tumors originate in the luminal cells, the milk-producing cells of the mammary gland. 10 percent of the cells resemble those of the underlying (basal) cell layer. These are muscle-like epithelial cells that contract when the mammary gland produces milk in order to push it out.