CXCR4 and B-cell chronic lymphocytic leukemia: This was also evident after stimulation with CXCL12, a ligand of CXCR4 that promotes sCXCR4 receptor internalization.12 In CLL patient cells, the overexpression of sCXCR4 correlates with hyper‐phosphorylated Ser339 residues.49 We observed that CUDC‐907 importantly decreased the phosphorylation level of CXCR4 at Ser339 (Figure 3D).