Inclusion Criteria:  - Evidence of treatment with a CDK4/6 inhibitor regardless of the line of therapy.  The initiation of the first CDK4/6 inhibitor was defined as the index date, and the first CDK4/6 inhibitor initiated was defined as the index treatment  BC diagnosis: Two diagnosis codes of BC (International Classification of Diseases, 9th Revision, Clinical Modification [ICD-9-CM]: 174.xx and International Classification of Diseases, 10th Revision, Clinical Modification [ICD-10-CM]: C50.xx [excluding C50.x2 - male BC]) on two medical service claims separated by at least 30 days Metastatic disease diagnosis: At least two medical claims for a secondary neoplasm (ICD-9-CM codes: 196.xx-197.xx, 198.xx, ICD-10-CM codes: C77.xx, C78.xx, C79.xx) on separate dates, with the first one occurring no more than 30 days before the first diagnosis for BC HR+/HER2-: At least one prescription fill or administration of an ET (anastrazole, exemestane, ethinyl estradiol, fulvestrant, fluoxymesterone, letrozole, megestrol acetate, tamoxifen, or toremifene), HR+/HER2- therapy (everolimus), or CDK4/6 inhibitor (i.e., abemaciclib, palbociclib, or ribociclib) at any time following the diagnosis of BC, and no claims for treatments indicated for HER2+ BC, including trastuzumab, lapatinib, afatinib, pertuzumab, or ado-trastuzumab, at any time in the data period Women of at least 18 years of age as of the index date At least 6 months of continuous health plan coverage prior to and at least 1 month of continuous health plan coverage after the index date  Exclusion Criteria:  None