Patients with IKAROS haploinsufficiency (HI) or haploinsufficiency of the dimerization domain (DD) present with progressive B-cell deficiency, whereas IKAROS dominant-negative (DN) results in more severe manifestations involving B-, T-, and myeloid cell defects (collectively termed as IKAROS deficiency) (3–6). Here, IKZF1 is linked to B cell deficiency.