Prospective Registry of ADC as First- and Second-line Treatment for Breast Cancer

Inclusion Criteria: 1. Male or female patients aged 18 years or greater with ability to provide written informed consent for this prospective registry study. 2. Estimated life expectancy of at least at 3 months per investigator assessment. 3. Willingness to provide an archival tissue sample and blood samples (20cc research blood collection at several timepoints) for research purposes. 4. Cohort-specific enrollment criteria: * Cohort 1: Histologically documented HR+/HER2- MBC with plan to start an FDA-approved ADC as their first ADC per standard of care (SOC). * Cohort 2: Histologically documented metastatic TNBC with plan to start an FDA-approved ADC as their first ADC per standard of care * Cohort 3: Histologically documented HR+/HER2- MBC with plan to start an FDA-approved ADC as their second ADC per standard of care (ADC1 should be an approved ADC administered per SOC or as monotherapy in a clinical trial; no prior experimental ADCs allowed). Clinical data from the first ADC must be available for retrospective review. * Cohort 4: Histologically documented metastatic TNBC with plan to start an FDA-approved ADC as their second ADC per standard of care (ADC1 should be an approved ADC administered per standard of care or as monotherapy in a clinical trial; no prior experimental ADCs allowed). Clinical data from the first ADC must be available for retrospective review. * Measurable disease is not required for any cohort. Exclusion Criteria: 1. Prior receipt of an experimental ADC in the metastatic setting. Of note, patients who received an FDA-approved ADC as their first ADC (as monotherapy, not in combination) can participate in cohorts 3 or 4 prior to starting their second FDA-approved ADC per standard of care. Of note, for all cohorts, experimental therapies are not allowed as intervening therapies after starting ADC1. If a patient enrolls on a clinical trial of an experimental therapy after ADC1, they will be taken off study. 2. Current participation in a clinical trial with an ADC. 3. Contraindication to research phlebotomy to collect \~20cc blood at each research blood draw timepoint.