In total, 64 patients with a HER2-negative primary tumor were screened for the presence of HER2-positive CTCs, of which 8 patients (7 ER + , 1 triple-negative breast cancer or TNBC) matched this criterion and proceeded to the treatment phase, where they received additional trastuzumab (flowchart Fig. 1). Here, ERBB2 is linked to neoplasm.