Anti-CD19-CAR–expressing T cells were detected in the blood of patients. (A) qPCR was used to determine the percentage of total PBMCs that contained the anti-CD19-CAR gene. The highest percentages of PBMCs containing the CAR gene were generally found between 6 and 14 days after infusion. (B) The absolute number of PBMCs containing the CAR gene per microliter of blood was determined by qPCR. (C) PBMCs from patient 8 were stained with a CAR-specific monoclonal antibody. Pretreatment PBMCs and PBMCs from 9 days after infusion were stained. Most CAR⁺ cells detected 9 days after infusion were CD4⁺. (D) PBMCs from patient 9 were stained with a CAR-specific monoclonal antibody. Pretreatment PBMCs and PBMCs from 7 days after infusion were stained. CAR⁺ cells included both CD4⁺ and CD8⁺ T cells. The plots in panels C and D are gated on live CD3⁺ lymphocytes.