Blocking of the CXCL12/CXCR4 signaling axis leads to the proliferation of primitive hematopoietic stem cells [51] while excessive signaling by CXCR4 accounts for the molecular pathoetiology of the WHIM syndrome [52] and is associated with tumor growth and metastasis formation [53]. Here, CXCR4 is linked to WHIM syndrome.