There is a way to head off resistance before it occurs. By reading the genetic codes of viruses as they mutate and evolve in actual patients, Robert M. Lloyd Jr., scientific director at Applied Sciences Inc. in Norcross, Ga., made a critical discovery. Before a strain of virus actually mutates enough to become resistant, he found, it typically makes an intermediate change in its genetic code. That mutation, Llyod reasoned, can be used to predict later mutations that lead to resistance.