 Trastuzumab, a monoclonal antibody to HER2, has improved the 5-year disease free survival of stage 1–3 HER2-positive breast cancers to about 87% (overall survival 95%).Between 25% and 30% of breast cancers overexpress the HER2 gene or its protein product, and overexpression of HER2 in breast cancer is associated with increased disease recurrence and worse prognosis.Trastuzumab, however, is very expensive, and its use may cause serious side effects (approximately 2% of people who receive it develop significant heart damage).