X-ray crystal structures of the ligand-bound and free forms of the variable domains of a catalytic antibody and its germline precursor. The binding of ligand (yellow) to the germline antibody (blue and purple) results in structural changes that lead to improved complementarity. The mutations in the antibody that occur as an immune response progresses (green and red) lead to a high-affinity, lock-and-key binding interaction. See 1665 and the Perspective on 1658. [Image: G. Wedemayer, L. Wang, P. Patten, P. Schultz, and R. Stevens]