Herceptin drug and breast cancer cell. Molecular model of Herceptin (Trastuzumab), showing the molecule's secondary structure, seen as alpha helices (coils) and beta sheets (arrows). A breast cancer cell is at lower left (pink). Herceptin binds to HER2 (human epidermal growth factor receptor 2), a receptor that is over-expressed in some breast cancer cells, making them grow faster and more likely to spread. Herceptin blocks HER2, which kills the breast cancer cells. Around a fifth of breast cancers result from cells that over-express HER2, making them viable for Herceptin treatment.