The HER2 protein, a member of the epidermal growth factor family, is encoded by the protooncogene c-erbB-2. Its overexpression, occurring in approximately one-third of all breast carcinomas, is associated with a poor prognosis. A humanized mouse antibody against HER2 has been developed by genetic engineering. Here an unspecific human IgG was connected to the recognizing mouse IgG fragment. The allergization typical for allogeneic antibodies does not take place in this context. The effectiveness of this antibody has been confirmed by two international prospective phase III trials that tested it alone and combined with chemotherapy. Both modes of application increased the response rates and the time to progression. Side-effects were rare except for a high rate of cardiac dysfunction when the antibody was combined with anthracyclines. The effectiveness and negligible side-effects of the chimeric antibody against HER2 (Herceptin) render it a valuable tool in the treatment of breast cancer.