BCR and B-cell chronic lymphocytic leukemia: CXCR4 surface expression and recycling are regulated by PIM (provirus integration site for Moloney murine leukaemia virus) kinases, which phosphorylate CXCR4 on serine 339.9 PIMs have been postulated as a key mechanism downstream of BCR, responsible for modulation of CXCR4 in CLL.8, 10 The family of PIM proteins involves three conserved oncogenic serine/threonine kinases, PIM1, PIM2 and PIM3.