It is believed that cytochrome P450 specificity is determined by a specific set of protein fragments that form the substrate binding site and are responsible for a particular orientation of the bound substrate relative to the activated oxygen atom. The essential roles of individual amino acid residues of the substrate binding pocket have been heavily studied. By using P450 BM-3 as a model, we have used scanning chimeragenesis to study the relationship between the fragments which form the binding pocket for the substrate. By using this technique, we have produced 20 functional chimeric proteins. Based on the activity assay results between chimeric proteins and the substrates lauric, palmitic acid and farnesol, we have produced several more catalytically active chimeric proteins with new regio-specificity and substrate selectivity.