To explore the therapeutic efficacy of PTPN2‐deficient HER‐2‐targeting CAR T cells in vivo, we adoptively transferred a single dose (6 × 106) of purified Ptpn2fl/fl versus Lck‐Cre;Ptpn2fl/fl central memory CD8+ HER‐2 CAR T cells into sub‐lethally irradiated syngeneic recipients bearing established orthotopic tumours arising from the injection of HER‐2‐expressing E0771 (HER‐2‐E0771) mammary tumour cells (Fig 4). Here, CD8A is linked to breast cancer.