An experimental approach to identification of the gene responsible for a given enzymic activity. Each open reading frame (ORF) of an organism, yeast for instance, is fused to the cDNA of glutathione S-transferase (GST) in a plasmid and then introduced into yeast. Yeast strains containing these plasmids are assayed in groups for the desired activity until the one strain with the activity is identified. (GST facilitates purification of the GST-ORF fusion protein by affinity chromatography.) Once the strain containing the activity is known, the ORF in the plasmid, which is responsible for the activity, is easily determined.