HER2 is a protein that sits on the surface of breast cancer cells in some people. Because you are one of these people, your breast cancer is called "HER2-positive." The HER2 protein is involved in the growth of your breast cancer. Certain drugs can interfere with the ability of the HER2 protein to cause breast cancer growth. Trastuzumab is one of these drugs. You must have already received trastuzumab as treatment for your breast cancer to be considered for this study.