CD19 and precursor B-cell acute lymphoblastic leukemia: Sotillo and colleagues showed a CD19 isoform that skipped exon 2 (Δex2) characterized by the loss of the cognate CD19 epitope necessary for anti-CD19 CAR-T cells is strongly enriched compared to prior anti-CD19 CAR-T cell treatment in some patients with B-ALL who relapse after anti-CD19 CAR-T cell infusion.