Description of species included in model and steady-state values in the absence of ligand.

*Values correspond to steady-state conditions in single-expressing cells where the total number of cell surface and intracellular receptors is 6×105 and 1.5×106 receptors/cell for CXCR4 and CXCR7, respectively. The total number of β-arrestin 2 molecules is 5.0×105 and 7.5×105 molecules/cell for endogenous β-arrestin 2 and β-arrestin 2-CBC, respectively. Receptor numbers are based on reasonable agreement with the data in Table C in File S1, the assumption that a large portion of the receptors are intracellular in the absence of ligand, and ability to fit internalization data (Fig. 3 E,F). β-arrestin 2 numbers are based on our data suggesting that the ratio of probe-labeled/endogenous β-arrestin 2 is ∼ 1.5 (Fig. 2A) and literature data (12).
**Values correspond to steady-state conditions in co-expressing cells where the total number of cell surface and intracellular receptors is 6×105 and 1.5×106 receptors/cell for CXCR4 and CXCR7, respectively. The total number of β-arrestin 2 molecules is 5.0×105 and 7.5×105 molecules/cell for endogenous β-arrestin 2 and β-arrestin 2-CBC, respectively.