Selective gene amplification.

We describe a system for directed evolution based on in vitro compartmentalisation in which amplification of a gene is coupled to the formation of product by the enzyme it encodes. This approach mimics the process of natural selection; 'fitter' genes--encoding more efficient enzymes--have more 'offspring'. It allows selection for any activity so long as a product-specific ligand (e.g. an antibody) is available.