Planned study population consists of approximately 240 adult patients with HER2-positive mBC receiving anti-HER2 therapy in 12 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.

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 January 2022 to 31st December 2023
• Patients with the availability of at least 24 months of follow-up data (from the date of mBC diagnosis) in the medical records at the participating site, unless patient died/moved or refused the therapy within the first 24 months of diagnosis
• Age ≥18 years at the time of inclusion

Exclusion Criteria:

• Presence of other malignancies within the period from mBC diagnosis until the timepoint of data collection
• Patients received trastuzumab deruxtecan
• The participation in any interventional trial within period since diagnosis until the end of study

This non-interventional multicenter observational study will include 240 adult patients with HER2-positive mBC receiving anti-HER2 therapy (except for trastuzumab deruxtecan).

Patients will be included in about 12 oncological centers (in each center it is expected to recruit about 20 patients) in various regions of Russian Federation

Written informed consent is not needed for this study due to its retrospective nature. The protocol will be reviewed by Central Ethics Committee (CEC). CEC approval will be obtained before the start of patients' data collection.