CUSTOM-MADE DRUG, the multicolored structure at the center, is a highly potent inhibitor of the enzyme purine nucleoside phosphorylase (PNP), represented by blue spheres. The inhibitor, developed using the structure-based drug design method, was created specifically to fit snugly in the enzyme's active site. It is now being tested in animals for the treatment of arthritis, and a closely related compound is in clinical trials as a therapy for psoriasis and for one form of cancer.