In this issue of Blood, Walters and colleagues describe an elegant model of WHIM syndrome in the zebrafish embryo. By allowing the movement of WHIM neutrophils to be observed in live animals, this model dramatically illustrates the dynamics of the interaction between the neutrophil chemokine receptor CXCR4 and its receptor ligand (stromal-derived factor-1 [SDF-1], also known as CXCL12) in th hallmark of WHIM-excessive neutrophil adhesion to the marrow stroma.