Abstract
Antibodies are critical reagents in biological research and are increasingly being developed as therapeutic agents. They typically exhibit very high affinity and selectivity for their ligands. Synthetic protein-binding agents rarely achieve the combination of high affinity and selectivity for their target protein that is typical of a good antibody. However, significant efforts are underway to develop a new generation of protein ligands with improved properties. This article reviews progress towards this goal and suggests fruitful strategies for future research.