To investigate if the antitumor activity of CD19-ENG T cells depended on significant T-cell expansion we injected BV173 leukemia-bearing mice on day 7 with 1 × 107 CD19-ENG or EphA2-ENG T cells that were genetically modified with a 2nd retroviral vector encoding eGFP.ffLuc (Supplementary Fig. 3). Here, EPHA2 is linked to leukemia.