The purpose of this study is to understand treatment adherence and patient-reported outcomes of switching to giredestrant due to prior aromatase inhibitor (AI) intolerance. Giredestrant will be administered as adjuvant endocrine therapy for participants with low-, medium-, and high-risk, Stage I-III, histologically confirmed, estrogen receptor positive (ER+), human epidermal growth factor receptor 2 negative (HER2-), early breast cancer (eBC), as defined by the investigator. Participants will enroll if considered to be intolerant to a prior adjuvant AI therapy.

Inclusion Criteria:

• Considered appropriate for treatment with endocrine therapy (ET)
• Histologically confirmed diagnosis of ER+/HER2-, Stage I-III (low-/medium-/high-risk) early breast cancer (eBC)
• Documented ER+ tumor according to American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP), defined as ≥1% of tumor cells stained positive
• Documented HER2- tumor according to ASCO/CAP
• Postmenopausal females at the time of signing the Informed Consent Form
• Documented use of a prior adjuvant aromatase inhibitor (AI) (i.e., anastrozole, exemestane, or letrozole) for a total of ≥6 months
• Documented use of an adjuvant AI (i.e., anastrozole, exemestane, or letrozole) for the consecutive ≥3 months immediately prior to consent
• Participant and investigator agree that current symptoms on AI are intolerable and warrant a switch in therapy to attempt sustained treatment
• Documented Grade 2 or 3 adverse events, per NCI CTCAE v6.0, determined by the investigator to be associated with AI therapy's intolerance
• Participant and investigator planning the first switch from an AI
• Has completed the following: (neo)adjuvant chemotherapy (if administered), definitive surgery of primary breast tumor(s) and/or axillary lymph nodes dissection (ALND) and/or sentinel lymph node biopsy (SLNB) and/or radiotherapy
• Eastern Cooperative Oncology Group Performance (ECOG) Performance Status 0 or 1

Exclusion Criteria:

• Participation within 6 months before enrollment in any other clinical study involving an investigational adjuvant treatment including anti-cancer agents
• Diagnosis of rheumatoid arthritis, psoriatic arthritis, or other inflammatory connective tissue disease
• Any prior fulvestrant or any oral selective estrogen receptor degraders (SERDs)
• Have active cardiac disease or history of cardiac dysfunction
• Have clinically significant liver disease consistent with Child-Pugh Class B or C, including active hepatitis (e.g., hepatitis B virus [HBV] or hepatitis C virus [HCV]), current alcohol abuse, cirrhosis, or positive test for viral hepatitis
• Treatment with strong CYP3A inhibitors or inducers within 14 days or 5 drug elimination half-lives (whichever is longer) prior to initiation of study treatment
• Have had any serious medical condition or abnormality in clinical laboratory tests that, in the investigator's judgment, precludes an individual's safe participation in and completion of the study

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing