GRK3 and WHIM syndrome: Given that GRK3 contributes to the CXCL12-promoted phosphorylation at S346/347 and that GRK3 prefers acidic residues N-terminal to the phospho-site [40], we also generated a HA-tagged E343K mutant (HA-CXCR4-E343K, Figure 6A), which corresponds to a recently discovered WHIM syndrome-associated mutation [30].