This is a special class of drugs used to block specific targets on the breast cancer cells. For example- approx 25% of the breast cancer patients have a change in the gene for the human epidermal growth factor receptor 2 (Her 2 neu) protein, which causes multiple copies of the gene to produce excessive number of Her 2 neu proteins in the cells. This causes rapid growth of cancer cells. Monoclonal antibodies work by attaching to the protein on the cancer cells like a key in the lock that in turn kills the cancer cells.