ERBB2 and neoplasm: It cannot be ruled out, however, that undetectable numbers of tumor cells may still be present in HER2 CAR T cell-treated brains and in one mouse we detected some residual HER2 + tumor cells at 6 weeks post CART cell administration (Figure 5E) and due to the termination of experiments after 7 weeks due to graft versus host disease, long-term outgrowth of tumor cells cannot be ruled out.