CD19 and precursor B-cell acute lymphoblastic leukemia: On the other hand, the major reason for treatment failure of the most advanced CAR T cells targeting CD19 is loss of antigen expression or selection by B-cell acute lymphoblastic leukaemia cells for mutants or variants that cannot be recognised by the anti-CD19 CAR T cells.82 Accordingly, highly specific antigen targeting by CAR T cells is accompanied by an inherent dilemma that applies selective pressure for the development of variants which are responsible for the relapse.