Although CXCR4 mutation in WHIM syndrome was not associated with any NK cell-related disease, a selective defect of CXCR4 internalization after CXCL12 binding underlies a new rare immune deficiency documented in two cases of disseminated Mycobacterium avium infection, where a marked reduction in the number of circulating NK cells as well as neutrophils and B cells was observed (66). The gene discussed is CXCL12; the disease is WHIM syndrome.