Figure 1. Targets for chimeric antigen receptors. (A) Ideally, targets for chimeric antigen receptors (CARs) should be expressed on all malignant cells, including immature cells with a high disease-initiating potential, to avoid the clonal escape of cancer cell subsets that do not express the CAR-targeted antigen. (B) The B-cell lineage antigen CD19 is expressed on malignant B cells or B-cell precursor cells in chronic lymphocytic leukemia (CLL) and acute lymphoblastic leukemia (ALL), respectively. In ALL patients, leukemia-propagating cells have also been found in the CD19^- B-cell compartment. These cells can drive both CD19^- and CD19⁺ relapses after the adoptive transfer of T cells expressing a CD19-specific CAR.