To this end, we generated and tested cellular models of WHIM syndrome to (i) compare side-by-side the in vitro functional activity of pathogenic CXCR4WHIM variants; (ii) evaluate each variant’s in vitro response to mavorixafor, a CXCR4 antagonist currently in clinical trials for the treatment of patients with WHIM syndrome and WM carrying CXCR4 GOF mutations [15]; and (iii) explore genotype-function-phenotype correlations in patients with WHIM syndrome. The gene discussed is CXCR4; the disease is WHIM syndrome.