Basis for therapeutic window. A therapeutic window can either be target-driven or context-driven. In target-driven drug design (a), the target molecule is unique to the diseased tissue. The context-driven strategy (b), on the other hand, is directed at a target that is present in both normal and diseased tissue. This alternative approach takes advantage of mutations or other physiological changes (hatched marks) that occur during tumorigenesis and that alter the cell’s or tissue’s requirement for the target protein. Such changes may include quantitative effects, as when a tumor cell becomes dependent on high expression of the target protein, or they may be qualitative, if the target protein takes on novel functions that sustain the growth of the diseased tissue.