PathHunter® eXpress Total GPCR Internalization cells are engineered to co-express the Enzyme Acceptor (EA) tagged GPCR, and a ProLink™ (PK) tag localized to the endosomes. Activation of the GPCR-EA induces internalization of the receptor in PK-tagged endosomes, forcing complementation of the two β-galactosidase enzyme fragments (EA and PK). The resulting functional enzyme 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.