The cloned enzyme donor immunoassay (CEDIA) refers to an immunoassay which uses a genetically-modified β-galactosidase enzyme. This enzyme has two units; an enzyme acceptor and an enzyme donor. These units alone are inactive, but in solution, they become activated and reassemble. As a single unit, they can react with the substrate and produce a change in absorbance. In the CEDIA, reagent antibodies keep the enzyme subunits apart, by physically blocking binding of the monomer subunits from forming a tetramer. If there is drug present in a patient sample it will bind to the antibody and 'overwhelm' it, allowing the monomers to bind and enzyme activity to ensue. See figure below for a depiction on the assay's mechanism of action.