CXCR4 and WHIM syndrome: The genetic cause of WHIM syndrome is heterozygous, gain-of-function mutation of the panleukocyte chemokine receptor CXCR4.9–11 This causes myelokathexis (myeloid hyperplasia, dysmorphic bone marrow neutrophils and severe neutropenia), resulting from an exaggeration of two normal CXCR4-dependent functions: nascent neutrophil retention in bone marrow and senescent neutrophil homing from blood to bone marrow.5 Hypogammaglobulinemia is incompletely penetrant.12