CXCR4 and myeloproliferative disorder: In addition, MDSCs are capable to migrate from the peripheral blood toward the spleen, and hereby expanding, thanks to their expression of CCR2 and CXCR4 and hence their ability to respond to CXCL12 and CCL2 gradients produced by splenic endothelia in emergency myelopoiesis or MPN.20, 22