ERBB2 and neoplasm: To determine whether nonengineered tumor lines express sufficiently high levels of tumor-associated surface antigens, we quantified HER2 and EGFR receptor levels on three human cell lines (Extended Data Fig. 8a) and targeted the split system to HER2 and EGFR (αHER2-Neo2A + αEGFR-Neo2B) (Extended Data Fig. 8b).