Illustration of anti-cancer monoclonal antibodies trastuzumab (red) and pertuzumab (orange) inhibiting human epidermal growth factor receptor 2 (HER2, light blue) proteins on a human cell membrane (purple and orange). 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.