From the time it was approved in 1998, Genentech's Herceptin--a drug in the vanguard of the first generation of so-called targeted therapeutics--has achieved an impressive track record for a subset of breast cancer patients. Some patients who take it live longer and the size of their tumors is kept under better control than if they had received standard chemotherapy alone.

To develop Herceptin, researchers at Genentech drew on investigations into the molecular workings of a cancer cell. Some breast cancer cells stud their exterior with a surfeit of receptors that join in pairs to trigger a cascade of signals that cause the cells to replicate uncontrollably, develop resistance to chemotherapy and encourage the growth of blood vessels that promote the spread of tumor cells.