This study will evaluate mechanisms of resistance to anti-breast cancer therapies in tumor and blood samples from participants with human epidermal growth factor receptor (HER2) positive, hormone receptor (HR) positive or triple negative breast cancer.

General Inclusion Criteria:

• Willingness to undergo a procedure to obtain tumor tissue (e.g. biopsy) and blood draw
• Diagnosis of HER2+, HR+ (for cohort R1) or triple negative breast cancer (for cohort T1) as per local assessment
• Availability of an archival tumor tissue (most recent pre-treatment tumor tissue is preferred)
• Unequivocally growing tumor lesion (progressive lesion) that is accessible for resection, excision or core needle biopsy
• Discontinuation of prior anti-cancer treatment outlined below should not be longer than 4 weeks from participation in this study

Inclusion criteria for participants in the cohorts studying acquired resistance

• Participant had undergone regular monitoring for disease progression as per local practice (preferably every 3-6 months) while on most recent breast cancer therapy
• Accessible tumor lesion that newly appeared or a lesion that started to regrow while the participant was at least 6 months on therapy

Inclusion criteria for participants in the cohort studying primary resistance

• Accessible tumor lesion that continued to increase in size or a newly appearing lesion (as confirmed by routine tumor assessment) while treated for at least 4 weeks but less than 6 months on therapy

Exclusion Criteria:

• Any risks factors that increase the risk of complications associated with the procedure to obtain tumor tissue (e.g. bleeding disorders)
• Any serious medical condition or abnormality in clinical laboratory tests that precludes an individual's safe participation in and completion of the study
• Participant has started treatment with subsequent anti-cancer therapy
• Participants whose progressive tumor lesion that is targeted for biopsy/resection is in the bone
• Discontinuation of treatment was due to a reason other than disease progression