Two transgenes controlled by the tetO (responsive to tet-transactivator) promoter, one containing "Ro1" and the other containing lacZ, were co-integrated so that beta-galactosidase could be used as a marker for expression. Ro1 is a prototype RASSL (Receptor Activated Solely by a Synthetic Ligand) with an N-terminal FLAG epitope that is extra-cellularly expressed and can be detected with the FLAG monoclonal antibody. Ro1 (RASSL, opioid, #1) is derived from a Kappa Opioid Receptor (G protein-coupled receptor) that was altered to reduce the activation by endogenous hormones, but preserves activation by an exogenous drug, spiradoline, allowing experimental control of G protein signaling.