TNFRSF9 and neoplasm: Furthermore, IL13-28ζ CAR T cells appeared more sensitive to lower levels of plate-bound IL13Rα2 and IL13Rα1, as evidenced by quantification of 4-1BB and CD69 tumor flux, which showed that IL13-28ζ had a lower activation threshold, and, unlike IL13-BBζ CAR T cells, exhibited off-target activity against IL13Rα1.