The siglec CD22 consists of 7 extracellular IgG-like domains and is expressed on B-cell leukemias and lymphomas. CAR T cells using different binding sites via the incorporation of single-chain variable fragments VH and VL derived from different anti-CD22 antibodies (m971 and HA22SH) and co-transduced with 2 co-stimulatory molecules (CD28 and CD3ζ) differ in their in vitro and in vivo antileukemic activity against CD22-positive target cells. CAR cells incorporating a more membrane-proximal epitope on CD22 (m971-28z) had a significantly higher in vitro activity and were more effective in reducing the leukemic burden of mice in a xenograft model and prolonging their survival compared with the CARs expressing the more distal binding site (HA22SH-28z). The red color of the mice in the insert is a measure for their leukemic burden at day 3 prior to injection of m971 or HA22SH CAR T cells and at days 7 and 15 after injection. Parts of the figure were adapted from Figure 6A in the article by Haso et al that begins on page 1165.