DiscoverX cell-based assays feature the powerful in vivo application of the established Enzyme Fragment Complementation (EFC) technology. In this approach, two complementing fragments; the small enzyme donor (ED) fragment and a larger enzyme acceptor (EA) fragment of the β-galactosidase (β-gal) enzyme are stably expressed as protein fusions in transfected cell lines. When ED binds to EA, complementation (EFC) occurs and β-gal activity is reconstituted and measured by substrate conversion to a luminescent product. The pCMV-ProLabel® and pCMV-ProLink™ mammalian cloning vectors are intended for mammalian cell expression of a GPCR or other protein of interest as a fusion protein with an ED [ProLabel (PL) or ProLink (PK)] tag. The resulting ED-tagged protein, upon interacting with an EA-tagged protein (most typically co-expressed in a EA-parental cell line), gives rise to EFC. Different ProLink and ProLabel tags have different affinities for the EA protein (and in the case of ARM-Scontaining vectors, different affinities for Arrestin) and their selection and usage are described in detail in the pCMVProLabel and pCMV-ProLink Mammalian Cloning Vectors User Manual.