This is an observational, multi-centre study analysing data from patients treated with T-DM1 within the compassionate use program AL41711. During the compassionate use program AL41711, subjects received T-DM1 treatment for a total of 14 cycles or less, in case of disease recurrence or unmanageable toxicity. After the completion of the treatment, and 90 days of wash-out period, they can be enrolled in the study. After the signature of the Informed Consent Form, all their data reported in the medical charts up to then (as detailed in section 12 of the protocol) will be extracted retrospectively and inserted in the study database. From the end of the washout period following the last treatment with T-DM1, each subject will enter in an observation phase for a maximum of 3 years, or until death whichever happens first. This phase will be prospective after the signature of the Informed Consent Form.

Inclusion Criteria:

• Signed written informed consent
• Patients who have received T-DM1 treatment within the compassionate use program AL41711 and have completed the entire treatment for at least 90 days or have interrupted it prematurely, due to disease recurrence or unmanageable toxicity, for at least 90 days, corresponding to the wash out period;

Exclusion Criteria:

• NA

The study will include approximately 300 patients with HER2-positive breast cancer that do not reach a pathological complete response following neoadjuvant chemotherapy and anti-HER2 target therapy and received T-DM1 within the compassionate use program AL41711.