Protein-fragment complementation assay, or PCA, is a method for the identification of protein–protein interactions in biological systems. It is used in the field of proteomics. In the protein-fragment complementation assay, the proteins of interest ("Bait" and "Prey") are covalently linked at the genetic level to incomplete fragments of a third protein (known as the "reporter") and are expressed in vivo, Interaction between the "bait" and the "prey" proteins brings the fragments of the "reporter" protein in close enough proximity to allow them to reform and become the functional reporter protein. Typically enzymes which confer resistance to antibiotics, such as Dihydrofolate reductase or Beta-lactamase, or proteins that give colorimetric or fluorescent signals are used. When used in the context of the reconstitution of fluorescent proteins, it is often called the Bimolecular fluorescence complementation assay.