Herceptin breast cancer drug. Molecular model of the drug Herceptin (trastuzumab, pink and yellow) binding to the HER2 receptor (blue). HER2 (human epidermal growth factor receptor 2) is a receptor found on some breast cancer cells. Overabundance of HER2 makes cancer cells faster-growing and more likely to spread. Herceptin blocks these receptors by binding to them, greatly improving the chances of curing the cancer. Around 20% of breast cancers have HER2 over-expression, and are thus viable targets for the drug. In the UK in 2005, it was announced that all women with early breast cancer would be tested to see if their tumours would be receptive to Herceptin therapy.