The protein responsible for cystic fibrosis has been identified using an approach called "reverse" genetics. This approach relies on the chromosomal map position to direct the search for a disease gene, several novel cloning strategies to isolate the gene, and the gene's sequence to define the abnormal protein. Reverse genetics, because it does not require prior knowledge of the protein's biochemical function, has wide utility and is being used to define the defects in many single-gene disorders. This update presents the reverse genetics approach and uses cystic fibrosis to illustrate the principles involved.