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

Inclusion Criteria: * Has pathologically documented breast cancer that: 1. Is unresectable or metastatic 2. Has low-HER2 expression defined as IHC 2+/ISH- or IHC 1+ (ISH- or untested) 3. Has progressed on, and would no longer benefit from, endocrine therapy 4. Has been treated at least 1 prior line of chemotherapy in the metastatic setting * Has protocol-defined 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 breast cancer ever assessed with high-HER2 expression * Has previously been treated with any anti-HER2 therapy, including an antibody drug conjugate * 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