Multicenter, noninterventional, retrospective study, aimed to determine the prevalence of HER2-low and HER2\>0\<1+ by rescoring of archived IHC-stained FFPE slides for HER2 in patients previously identified as having HER2-negative locally-advanced or mBC who progressed on any systemic anticancer therapy.The study will also describe the baseline sociodemographic and clinicopathological characteristics, treatment patterns, clinical outcomes and concordance between historical IHC status and rescored IHC status of archived IHC-stained FFPE slides. The study will not have any study-specific patient visits, procedures, or a longitudinal follow-up. All available data will be extracted from patients' medical records. The study will consist of 2 components: rescoring of archived IHC-stained FFPE slides and secondary data collection from patients' medical records. Human epidermal growth factor receptor 2 IHC historical scores, HER2 rescoring results of archived IHC-stained FFPE slides by qualified laboratory (local and/or independent central laboratory), other biomarker testing results based on historical testing and/or testing of archived tissue samples when available. The data on different types of treatment received by the patients and sociodemographics and clinicopathological characteristics will be extracted from patients' medical records from the date of diagnosis of HER2-negative BC up to the date of data extraction.

The patients identified for the current study will be a convenience sample of all patients in the relevant electronic health record/electronic medical record databases and biobanks who meet the inclusion and exclusion criteria; no a priori power analysis will be conducted. The study aims to capture approximately minimal 150 to 200 HER2-negative mBC patients per participating country/cluster, in order to generate data on 2100 to 2700 patients overall, with the aim to identify approximately 1050 HER2-low patients (at least 600 patients in Asia cohort and 450 patients in Latin America \[LATAM\] cohort) for analysis, subject to revision based on preliminary data analysis. Clinical outcomes will be assessed as per the HER2 subsets identified after rescoring (HER2 low-IHC1+ or IHC2+/ISH-, HER2 IHC\>0\<1+, HER2 null, and HER2 zero-HER2 IHC\>0\<1+ and HER2 null). The study will be conducted in 8 Asian countries including Hong Kong, India, Indonesia, Malaysia, Philippines, Thailand, Singapore, and Vietnam and 6 Latin American (LATAM) countries including Argentina, Brazil, Chile, Dominican Republic, Mexico, and Panamá. Regions and countries may be added based on feasibility assessment as per AstraZeneca's standard operating procedures. A total of approximately 2100 to 2700 patients with a confirmed diagnosis of HER2-negative, locally-advanced or mBC regardless of HR status between 01 January 2019 and 31 December 2022 with the availability of medical records of at least 12 months of follow-up data (from the index date), and deemed eligible as per the study eligibility criteria will be enrolled in the study at approximately 28-58 study sites. The study protocol and informed consent form (ICF) will be approved by local Institutional Review Boards (IRBs)/(Institutional) Ethics Committees (IECs) before the commencement of recruitment.