CAR T cells recognize very low levels of CD19. (A) Expanded populations of CD19^– precursors (also CD22⁺CD24^–CD10 variable CD34 variable) that are seen after CAR T-cell therapy. (B) Flow cytometric estimation of CD19 expression (antigen-binding capacity) in CD19^– K562 cells transduced with increasing doses of CD19 mRNA (n = 3). (C) Significant CD69 expression in CAR T cells at 0.005 μg CD19 mRNA (n = 3). (D) Significant CD137 expression at 0.1 μg CD19 mRNA (n = 3). (E) Significant killing by CAR (19-BBz CAR construct transduced) at 0.02 μg CD19 mRNA at a 10:1 effector-to-target ratio (n = 3). Student t test; *P < .05, **P < .01, ***P < .001, and ****P < .0001.