Herceptin breast cancer drug. Molecular model of the drug Herceptin (trastuzumab). Overabundance of Her2, a receptor found on some breast cancer cells, makes them faster-growing and more likely to spread. Hereceptin 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. However, the decision has been controversial due to the divided medical opinions surrounding the effectiveness of the drug.