Herceptin breast cancer drug, molecular model. Herceptin (Trastuzumab, blue and yellow) is a monoclonal antibody that binds to the HER2/neu receptor (pink) on breast cancer cells, causing them to die during their reproductive cycle. Around a fifth of breast cancers are made up of cells that over-express this receptor, making them viable for Herceptin treatment. In these models, the secondary structures of the receptor and antibody are shown. Alpha helices are spirals, beta sheets are shown as arrows. The arrangement of these domains forms a protein's tertiary structure, which helps determine its function.