ERBB2 and neoplasm: The CirCe T-DM1 trial showed a very limited efficacy of T-DM1 in patients with pretreated HER2-negative mBC, and HER2 amplification found on CTC: a strong limitation of this approach is the need for detecting a sufficient number of CTC to test biomarkers of interest.68 On the other side, biomarker analysis on ctDNA is emerging as a tool to monitor tumor response and select patients for ADCs.69