CXCR4 and WHIM syndrome: In vitro assays using cell lines with exogeneous overexpression of CXCR4WHIM variants can model the GOF cellular phenotypes (Fig. 2 and related literature [8, 9, 17, 22, 25]), but may not entirely represent the situation in patients with WHIM syndrome, who are heterozygous and have one wild-type CXCR4 allele.