A review with 15 refs. Directed mol. evolution of enzymes and proteins has emerged as an extremely powerful method to create proteins with novel properties, both for practical applications as well as for mechanistic studies. To demonstrate the underlying principles of this approach, the authors describe here their work on the heme-contg. cytochrome c peroxidase (I) from Saccharomyces cerevisiae. Using directed evolution, the authors changed the substrate specificity of I from the protein, cytochrome c, to a small org. mol. with the best mutants possessing up to 300-fold higher activity against a phenolic substrate. In addn. to novel insights into the mechanism of peroxidases, the results illustrate the ability of directed mol. evolution technologies to deliver solns. to biochem. problems that would not be readily predicted by rational design. [on SciFinder (R)]