Roughly one in four breast cancers has too much of a protein called HER2, which makes the cancer grow faster. The medication Herceptin (trastuzumab) targets the HER2 protein to help breast cancer patients live longer. But this medication may be linked to heart problems.

Scientists have known for a while that some chemotherapy agents can cause heart disease in some patients. This complication is now being seen in some women taking a popular breast cancer drug.