More than 53,600 women are diagnosed with breast cancer ever year in the UK. And around 1 in 5 of these women have tumours that are classified as ‘HER2 positive’. That’s because the surface of each breast cancer cell is littered with extra copies of a molecule called the human epidermal growth factor receptor 2 (HER2). And this leaves the cells extremely sensitive to signals that cause them to grow and divide. The majority of women diagnosed with this type of breast cancer have treatment aimed at interfering with these signals, the most common of which is trastuzumab (Herceptin).