Since development of aggressive leukemia in mice with heterozygous deletions of the IK exons encoding the zinc fingers required for sequence-specific DNA binding occurs only with a concomitant loss of heterozygosity between 3 and 6 months after birth [19], we sought to determine if leukemic blast cells from aggressive early relapse patients are characterized by homozygous deletions of IKZF1 exons 4-7 involved in reported heterozygous IK deletions in high-risk BPL. The gene discussed is IKZF1; the disease is leukemia.