Summary: Activated GPCR Internalization cells are engineered to co-an untagged GPCR, Enzyme Acceptor (EA) tagged β-Arrestin, and a ProLink (PK) tag localized to the endosomes. Activation of the GPCR induces β-Arrestin recruitment, followed by internalization of the Receptor/Arrestin-EA complex in PK-tagged endosomes. This forces complementation of the two β-galactosidase enzyme fragments (EA and PK), forming functional enzyme that hydrolyzes substrate to generate a chemiluminescent signal. These cells have been modified to prevent long term propagation and expansion using a proprietary compound that has no apparent effect on assay performance.