Although the primary tumour was HER2-negative, amplification of a segment including the ERBB2 gene was detected in 12/15 CTCs analysed across the 2 blood samples and both CTC pools (Fig. 2d and Supplementary Tables 3 and 4), suggesting the patient had acquired ERBB2 amplification during adjuvant therapy and might therefore have responded to Herceptin if it had been administered. Here, ERBB2 is linked to neoplasm.