This is a multicenter observational study with both retrospective and prospective phase, designed to evaluate the clinico-pathologic characteristics and outcomes of patients with HR+/HER2- EBC at high risk of recurrence treated with ribociclib or abemaciclib in combination with ET in the adjuvant setting.

The study will also evaluate clinico-pathologic characteristics, patterns of recurrence, therapeutic choices and outcomes of the subgroup of patients relapsed during or within 12 months after adjuvant therapy with CDK4/6 inhibitors in combination with ET.

Inclusion Criteria:

• Female (regardless of menopausal status) or male ≥18 years of age;
• The patient has confirmed HR+, HER2-, early-stage resected invasive breast cancer candidate to adjuvant CDK4/6 inhibitors (abemaciclib or ribociclib);
• ER and PgR positivity is defined as evidence of immunohistochemical staining ≥ 1% according to ASCO/CAP recommendations; HER2 negativity is defined as expression of the membrane protein in immunohistochemistry 0 or 1+ or with a 2+ in situ hybridization (ISH) test negative as per ASCO/CAP recommendations;
• Patients who are initiating or have initiated adjuvant treatment with a CDK4/6 inhibitor (abemaciclib or ribociclib) in combination with endocrine therapy (ET). Treatment must have started on or after January 1, 2021.
• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2;
• Consent to data treatment according to the local regulation.

Exclusion Criteria:

• Current participation in any other HR+/HER2- EBC study with any investigational products;
• Patients who have already received adjuvant treatment with a CDK4/6 inhibitor as part of a clinical trial;
• Patients unable to comply with the requirements of the study or who, in the judge of the study physician, should not be included in the study;
• Patients with a history of previous BC, with the exception of Ductal carcinoma in situ (DCIS) treated by locoregional therapy alone ≥5 years ago;
• Patients with a history of any other cancer (except non-melanoma skin cancer or carcinoma in situ of the cervix), unless in complete remission with no therapy for a minimum of 5 years from the index date, will be excluded, as well.

All patients diagnosed with HR+/HER2- breast cancer who have received medical treatment in the neoadjuvant or adjuvant setting (chemotherapy, radiotherapy and hormonotherapy) and who have received or are scheduled to initiate adjuvant treatment with CDK4/6 inhibitors (abemaciclib or ribociclib) in combination with ET in the adjuvant setting.