Inclusion Criteria:

- Female patients aged 18 years or older at diagnosis of, or progression to, metastatic
breast cancer

- Patients with a diagnosis of breast cancer with documented metastatic disease and
with known date of metastatic disease

- Patients with a histologically confirmed HER2+ breast cancer (HER2 testing procedures
per routine institutional practice; no tissue re-sampling will be performed. If
conducted at another site, documented results must be available).

Exclusion Criteria:

- Patients receiving care for another primary cancer during the study time period.