Unless otherwise stated all chemicals and reagents were purchased from Sigma and Fisher. HEK293A WT, HEK293 WT, and CRISPR GRK and β-arrestin1/2-knockout cells were generous gifts from Aska Inoue (Tohoku Univ.)23 (link), 42 (link). Methoxy e-Coelenterazine (Prolume Purple) was purchased from Nanolight Technologies (Prolume LTD) and fluorescent antibodies from Li-COR Biosciences. CXCL12 ELISA kits were from R&D Systems.