Illustration of anti-cancer monoclonal antibodies (blue) trastuzumab and pertuzumab inhibiting membrane-bound human epidermal growth factor receptor 2 (HER2, red) proteins. HER2 belongs to the epidermal growth factor receptor (EGFR) family of proteins. It is involved in promoting cell proliferation and opposing apoptosis (programmed cell death). Over-expression of HER2 is implicated in a range of cancers, notably breast cancer. Monoclonal antibodies such as trastuzumab and pertuzumab can bind to HER2 receptors and inhibit them, closing the signalling pathways that promote cancer cell proliferation.