HER2 Expression of CTC to Predict Response in HER2-positive Advanced Breast Cancer Patients Treated With ADC

Inclusion Criteria: * Has pathologically documented breast cancer that: 1. Is unresectable or metastatic 2. has confirmed HER2-positive expression defined as IHC 2+/ISH+ or IHC 3+ 3. was previously treated with trastuzumab and taxane in the advanced setting or progressed within 6 months after neoadjuvant or adjuvant treatment involving a regimen including trastuzumab and taxane * Has adequate cardiac, bone marrow, renal, hepatic and blood clotting functions * Receives anti-HER2 ADC treatment * Life expectancy \> 3 months * Evidence of measurable or evaluable disease (RECIST 1.1 criteria)in the 28 days before selection * ECOG ≤2 Exclusion Criteria: * Has uncontrolled or significant cardiovascular disease * Has history of (noninfectious) interstitial lung disease (ILD)/pneumonitis that required steroids, has current ILD/pneumonitis, or suspected ILD/pneumonitis that cannot be ruled out by imaging at screening * Has any medical history or condition that per protocol or in the opinion of the investigator is inappropriate for the study