Inclusion Criteria

Inclusion criteria for Cohort 1:

1. Histologically proven AJCC TNM stage II-IV invasive breast cancer

2. HER2-positivity defined as score 3+ on IHC and/or with HER2/neu amplification on FISH
test (HER2/CEP17 ratio >2.0 on PathVysion test)

3. Evidence of complete response (CR) according to RECIST 1.1 criteria lasting for at
least 36 months (for stage IV patients only) following a first-line chemotherapy and
trastuzumab

4. Pathological Complete Response (pCR) following a neo-adjuvant chemotherapy (CT) and
trastuzumab (for stage II-III patients only). pCR is defined as no evidence of
residual invasive carcinoma in the breast (ductal carcinoma in situ (DCIS) is
allowed) AND in all the examined lymph nodes (micro-metastases and isolated tumour
cells are not allowed).

5. At least one FFPE archived tissue sample from the primary tumour and/or a metastatic
site(s) available for laboratory analyses

6. Adequate follow up information

Inclusion criteria for Cohort 2:

1. Histologically proven AJCC TNM stage II-IV invasive breast cancer

2. HER2-positivity defined as score 3+ on IHC and/or with HER2/neu amplification on FISH
test (HER2/CEP17 ratio>2.0 on PathVysion test)

3. Progression of disease according to RECIST 1.1 while receiving trastuzumab (in
association with chemotherapy or as single agent as maintenance therapy) or within 6
months from last dose of trastuzumab (for stage IV patients only)

4. Residual invasive tumour in the breast larger than 2cm and /or at least one micro- or
macro-metastasis in the axillary lymph nodes following pre-operative
trastuzumab-containing chemotherapy (for stage II-III patients only)

5. At least one FFPE archived tissue sample from the primary tumour and/or a metastatic
site(s) available for laboratory analyses

6. Adequate follow up information

Exclusion Criteria:

1. Any deviation from the above mentioned Inclusion criteria