Fusion protein offer an important biological assay for direct protein-to-protein interaction. For instance, to demonstrate that protein X binds to protein a GST-X fusion protein would be generated. Assay beads, coated with the tripeptide glutathione, strongly bind the GST fusion protein ( GST-X ). Therefor if X bind Y, then GST-X will also bind Y, and Y will be present on assay beads.