The crucial gatekeeper function of IKZF1 was further illustrated in two cases of identical twins with prenatal-acquired BCR-ABL1 fusion protein in a portion of their newborn B cells, where one of both twins in each case additionally acquired a postnatal IKZF1 deletion, which led to transformation into overt and fatal B-ALL (previously free of leukemia cells like the other twin) or to death after B-ALL disease (the other twin went into remission after treatment) of the respective twin [233]. Here, IKZF1 is linked to precursor B-cell acute lymphoblastic leukemia.