For many years scientists and cancer researchers have been trying to find out why some people benefit more from anti-cancer medications than other people who receive the same amount and same kind of medications. Current studies suggest that inherited characteristics might explain these differences. Height and eye color are examples of characteristics that have been inherited from parents. Studies suggest that people might also inherit genetic differences in how their bodies break down medications. When a person receives an anti-cancer medication, it is broken down by the liver into smaller parts or by-products. To try to understand more about how people's bodies break down anti-cancer medications, the researchers are studying the by-products (called metabolites) of epirubicin in the blood of people who are taking this medication as part of their breast cancer treatment.