We found that low levels of IKAROS result in increased CD19 mRNA with co-retention of introns 2 and 10 and confirmed the increased expression of this isoform in primary patient samples from CD19neg relapsed B-ALL and LBCL patients after blinatumomab13, 19.BBz11,14, and 19.28z29,30 CAR T therapies. Here, IKZF1 is linked to precursor B-cell acute lymphoblastic leukemia.