Chinese Hamster Ovary (CHO) PathHunter cells expressing enzyme acceptor (EA)-tagged β-arrestin 2 and ProLink (PK)-tagged DOR (CHO-OPRD1) were from DiscoveRx (Freemont, CA). Cells were grown in F-12 media (Invitrogen 11765), containing Hyclone FBS 10%, hygromycin 300 μg ml−1 (Invitrogen 10687), G418 800 μg ml−1 (Invitrogen 10131) and maintained at 37 °C in a humidified incubator containing 5% CO2. These cells were used for β-arrestin recruitment assays and inhibition of forskolin-stimulated cAMP accumulation assays described below.