CXCR4 and immune system disorder: Consistently, mutations in the human CXCR4 gene causing small C-terminal deletions of 10 to 19 residues or a single amino acid exchange (E343K) are associated with WHIM syndrome (a rare immunodeficiency characterized by warts, hypogammaglobulinemia, infections, and myelokathexis) and gain-of-CXCR4-function [27], [28], [29], [30].