The first full-scale trials of an experimental drug that attacks a genetic defect in breast cancer cells show significant promise for women with advanced cancer, scientists reported Sunday.

The drug, Herceptin, heightened the benefits of chemotherapy by shrinking tumors and slowing progression of a particular type of breast cancer, one that is responsible for about 30 percent of the 180,000 new cases of the disease in the United States each year, the scientists said at the annual meeting of the American Society of Clinical Oncology.Participants at the meeting greeted the reports with cautious optimism, and leaders said they were now planning to test the drug among patients with less advanced forms of the breast cancer and among the 20 percent of ovarian cancer cases involving the same genetic defect.