CXCR4 and WHIM syndrome: CXCR4 internalization (or desensitization) is critical for appropriate regulation of CXCR4 signaling, given that defects in its internalization maintain the receptor in a constitutively active form that causes an immune deficiency syndrome named Warts, Hypogammaglobulinemia, Infections and Myelokathexis (WHIM) syndrome in humans [15–18].