PIM1 and B-cell chronic lymphocytic leukemia: PIM kinases (predominantly PIM1) are the key regulators of CXCR4 (S339) phosphorylation, facilitating membrane retention of the receptor in myeloid and lymphoid cells.9, 42 Inhibition of PIMs with the newly developed SEL24‐B489 pan‐PIM inhibitor in primary CLL cells led to a time‐dependent decrease of S339 phosphorylation and concurrent decrease in CXCR4 surface membrane expression.