MKI67 and acute lymphoblastic leukemia: However, despite this sustained activation, the proportion of proliferating Ki‐67+ CLL‐derived CAR T cells was significantly lower compared to their ALL‐derived counterparts in the periphery of the 3D co‐culture (CD4+ CAR T cells: CLL 55.3% ± 8.4 vs. ALL 79.7% ± 2.6; CD8+ CAR T cells: CLL 60.9% ± 6.0 vs. ALL 83.4% ± 3.3; P < 0.05 Mann–Whitney U test, Figure 3C), indicating a reduced proliferative capacity of CLL‐derived CAR T cells.