"Breast cancer is the most common type of cancer among women in the United States, accounting for more than a quarter of all cancers in women. Approximately 2.5 million women in this country are breast cancer survivors, and an estimated 192,370 new cases of breast cancer were diagnosed in women in 2009. Advances in treatment have enabled more women to live longer overall and to live longer without disease progression. Yet not every treatment is effective for every woman. Why?