We have previously demonstrated that the overrepresentation of transitional B cells in the PB of patients with WAS is, at least in part, due to an early export of immature B cells from the BM because of the inability of B cells from patients with WAS to sense retention signals mediated by CXCR4 and its ligand, SDF-1α.18 Here, CXCR4 is linked to Wiskott-Aldrich syndrome.