Classically, studies on the relationships between structure and function have focused on the synthesis of individual molecular species and then analyzed their functions in cells. Recently, an entirely different approach has become feasible. A series of techniques has made it possible to simultaneously synthesize enormous numbers of related species of molecules and to select from these libraries specific molecules encoding unique activities (Fig. 1 ). Vast libraries of proteins (1 ,2 ), RNA (3 ), DNA (4 ), and small organic molecules (5 ) have been assembled. From these libraries molecules have been selected based on binding (6 ) and catalytic activities (7 ).
