Mice carrying the same gain-of-function mutation in the gene CXCR4 that is present in human patients suffering from the Warts, Hypogammaglobulinemia, Infections, and Myelokathexis (WHIM) syndrome were previously created to understand the biology underlying this syndrome and to develop better means for treating WHIM patients. The gene discussed is CXCR4; the disease is infection.