Planned study population consists of approximately 1,000 adult patients with HER2-positive mBC receiving anti-HER2 therapy in about 50 oncological centers (in each center it is expected to recruit about 20 patients) in different regions in order to provide representative study sample. Patients will be included consecutively from the least recent diagnosis (within defined time period).

Planned retrospective follow-up period for 1 patient is a period starting from the date of mBC diagnosis until end study or until patient's death, whichever occurs first. End of study will be at least 12 months after the latest date of mBC diagnosis to ensure all patients have the opportunity to contribute at least 12 months of data.

Planned study population consists of approximately 1,000 adult patients with HER2-positive mBC receiving anti-HER2 therapy in about 50 oncological centers (in each center it is expected to recruit about 20 patients) in different regions in order to provide representative study sample. Patients will be included consecutively from the least recent diagnosis (within defined time period).

Planned retrospective follow-up period for 1 patient is a period starting from the date of mBC diagnosis until end study or until patient's death, whichever occurs first. End of study will be at least 12 months after the latest date of mBC diagnosis to ensure all patients have the opportunity to contribute at least 12 months of data.

Only patients who were diagnosed between the 1st July 2021 and 1st July 2022 will be included. Patients should have at least 12 months of follow-up data. In case patient died within the first 12 months of diagnosis, these data from date of mBC diagnosis until patient's death should also be included. It will be multicenter study

Inclusion Criteria:

• Patients with HER2-positive mBC who are receiving or have started any anti-HER2 therapy (except for trastuzumab deruxtecan) at the time of inclusion;
• The diagnosis of mBC was established between the 1st July 2021 and the 1st July 2022;
• Patients with the availability of at least 12 months of follow-up data (from the index data) in the medical records at the participating site, unless patient died within the first 12 months of diagnosis;
• Age ≥ 18 years at the time of inclusion.

Exclusion Criteria:

• Presence of other malignancies within period since diagnosis until the timepoint of data collection;
• Patients receiving trastuzumab deruxtecan currently or in the anamnesis;
• The participation in any randomized controlled trial within period since diagnosis until end of study.

This non-interventional multicenter observational study will include 1,000 adult patients with HER2-positive mBC receiving anti-HER2 therapy (except for trastuzumab deruxtecan).

Patients will be included in about 50 oncological centers (in each center it is expected to recruit about 20 patients) in various regions of Russian Federation