Barnase and barstar are the extracellular ribonuclease and its intracellular inhibitor produced by Bacillus amyloliquefaciens. Both are small single-chain proteins and thus are suitable for application to the study of how a protein's sequence directs its fold. Barnase has neither disulfide bonds nor non-peptide components and unfolds reversibly in what closely approximates a two-state reaction. The genes for both these proteins have been cloned in E. coli. Expression of barstar is necessary to counter the lethal effect of expressed active barnase. Site-directed mutagenesis is being used to answer specific and general questions relating to protein folding and protein-protein interaction.