The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

Traditionally, large numbers of existing substances get tested until one is found that will produce a desired effect. Since disease involves the action of proteins, scientists are hoping to bioengineer drugs with specific molecular structures, tailoring them either to promote or inhibit the effect of proteins in the body.
A(an) （） for new drugs against diseases.