Trastuzumab (INN; trade name Herceptin) is a monoclonal antibody that interferes with the HER2/neu receptor. Its main use is to treat certain breast cancers.

The HER receptors are proteins that are embedded in the cell membrane and communicate molecular signals from outside the cell to inside the cell, and turn genes on and off. The HER proteins regulate cell growth, survival, adhesion, migration, and differentiation—functions that are amplified or weakened in cancer cells. In some cancers, notably some breast cancers, HER2 is over-expressed, and causes breast cells to reproduce uncontrollably.

Antibodies are molecules from the immune system that bind selectively to different proteins. Trastuzumab is an antibody that binds selectively to the HER2 protein. When it binds to defective HER2 proteins, the HER2 protein no longer causes cells in the breast to reproduce uncontrollably. This increases the survival of people with cancer. However, cancers usually develop resistance to trastuzumab.

The original studies of trastuzumab showed that it improved overall survival in late-stage (metastatic) breast cancer from 20.3 to 25.1 months, but there is controversy over whether trastuzumab is effective in earlier stage cancer. Trastuzumab is also controversial because of its cost, as much as $100,000 per year, and while certain private insurance companies in the U.S. and government health care systems in Canada, England and elsewhere have refused to pay for trastuzumab for certain patients, some companies have since accepted trastuzumab treatment as a covered preventative treatment.