CD19 and B-cell acute lymphoblastic leukemia: For instance, in pediatric B-cell acute lymphoblastic leukemia (B-ALL), CD19-negative relapse occurs in approximately 10-30% of patients post-CAR-T therapy due to CD19 downregulation or loss, often mediated by alternative splicing of CD19 mRNA or clonal selection of pre-existing CD19-negative subpopulations 12.