Breast Cancer RElapsed in Patients Treated With Adjuvant CDK4/6 Inhibitors:

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.