The authors propose a model whereby Arr3 competes with GRK2 for interaction with the phosphorylated receptor. Although our data are consistent with this model, we observed that Arr3 was recruited to NSC12 GLP-1R in one phase as opposed to two phases as observed by Jorgensen et al. It is possible that this difference is due to the different assays used to monitor the time course of GLP-1R/ Arr3 interaction. Single-cell FRET allows for greater temporal resolution than BRET, which also measures interactions in cell populations as opposed to single cells. The two phases are explained as phosphorylation-independent and -dependent arrestin recruitment. We did not detect an initial phosphorylationindependent phase for Arr3 recruitment to GLP-1R. A two-phase arrestin association has previously been observed for the b2adrenergic receptor, and an alternative UMI-77 explanation is that the first phase is due to arrestin recruitment to pre-phosphorylated receptors. The time course in our experiments was comparable to arrestin recruitment to the parathyroid receptor, which also displays a one-phase association. Again, in contrast to GLP-1R and in agreement with our previous experiments, GIPR stimulation did not result in either GRK2 or arrestin recruitment. GRK2 overexpression has been shown to increase agonist-mediated GIPR phosphorylation; however, this has not been demonstrated to be a direct effect. To our knowledge, a direct interaction between GIPR and GRK2 has only been demonstrated through immunoprecipitation assays in adipocytes. This difference may be due to the cell type or the method used to assess GIPR/GRK2 interaction. It is also possible that the addition of the YFP molecule to the C-terminus of GIPR prevents the receptor from interacting with GRK2 or Arr3. This possibility is unlikely, however, as YFP-labelled GLP-1R was able to interact with both GRK2 and Arr3, and the receptors share similar sequences. GPCRs can be classified by their interactions with arrestin. Class A receptors interact with arrestin transiently at the plasma membrane after agonist stimulation, whereas class B receptors cointernalise with arrestin.