In mice bearing orthotopic E0771-HER2 breast tumours or subcutaneous MC38-HER2 tumours, the overexpression of TCF7, FOXO1-ADA or ID3, but not JUN, significantly enhanced CAR T cell efficacy through more durable anti-tumour control, and a significant increase in survival for mice treated with FOXO1-ADA or ID3-expressing CAR T cells relative to controls (Fig. 1a–c and Extended Data Fig. 4a–d). The gene discussed is ID3; the disease is breast neoplasm.