A 23-year-old man presented with neutropenia and a history of warts from ages 8 to 10 years that resolved spontaneously until recurrence at age 20 years. Other than 2 cutaneous abscesses and recurrent otitis media limited to childhood, he denied severe or unusual infections. Scars from numerous prior warts were evident on the upper extremities. Previously, his absolute neutrophil count had been reported as 120/mm3. At the current examination, his IgG3 level was reduced, but the serum concentrations of IgA, IgM, and IgG were normal. Test results for neutrophil antibodies, antinuclear antibodies, human immunodeficiency virus, isoagglutinin titers, and antibody responses to diphtheria, influenza B, and pneumococcal vaccination were also normal. Bone marrow biopsy yielded an extremely hypercellular specimen with increased granulopoiesis that was discordant with the patient's neutropenia. Microscopic examination of the bone marrow aspirate showed complete neutrophilic maturation, but the nuclear lobes in the mature neutrophils were connected by abnormally long and stringy filaments of chromatin. Genomic DNA sequencing from peripheral blood mononuclear cells revealed a heterozygous point mutation, R334X, in the full-length transcript isoform b at the C-terminal resulting in a predicted premature truncation of the CXCR4 protein. WHIM syndrome is an autosomal dominant disorder characterized by warts, hypogammaglobulinemia, infections, and myelokathexis.1Gorlin RJ Gelb B Diaz GA Lofsness KG Pittelkow MR Fenyk JR WHIM syndrome, an autosomal dominant disorder: clinical, hematological, and molecular studies.Am J Med Genet. 2000; 91: 368-376Crossref PubMed Scopus (165) Google Scholar, 2Diaz GA CXCR4 mutations in WHIM syndrome: a misguided immune system?.Immunol Rev. 2005; 203: 235-243Crossref PubMed Scopus (71) Google Scholar Myelokathexis refers to the discordance between hypercellular bone marrow and neutropenia in peripheral blood3Krill CE Smith HD Mauer AM Chronic idiopathic granulocytopenia.N Engl J Med. 1964; 270: 973-979Crossref PubMed Scopus (59) Google Scholar, 4Zuelzer WW “Myelokathexis”—a new form of chronic granulocytopenia: report of a case.N Engl J Med. 1964; 270: 699-704Crossref PubMed Scopus (136) Google Scholar and is caused by an abnormality of CXCR4, a chemokine involved in cell trafficking and homing.5Hernandez PA Gorlin RJ Lukens JN et al.Mutations in the chemokine receptor gene CXCR4 are associated with WHIM syndrome, a combined immunodeficiency disease.Nat Genet. 2003; 34: 70-74Crossref PubMed Scopus (538) Google Scholar Further evaluation should be considered in patients with warts and neutropenia, particularly those with recurrent infections. We thank Drs Robert L. Phyliky, Roshini S. Abraham, and David P. Steensma for the clinical and laboratory evaluations that contributed to the findings in this case.