• Signed written informed consent.

• Age ≥18 years.

• Histologically or cytologically confirmed HER2-positive or borderline positive (for definition see below) adenocarcinoma of the breast.

• HER2-positive defined as:

‣ 3+ by immunohistochemistry \[IHC\] in \>10% of cell areas.

‣ 2+ by IHC in \>10% of cell areas and HER2/CEP17 ratio ≥2.0 or HER2 copy number ≥ 6.0 by in situ hybridization \[ISH\]).

∙ HER2-borderline positive defines as:

‣ 2+ by IHC and HER2/CEP17 ratio \<2.0 and/or HER2 copy number 4.0 - 6.0 by in situ hybridization \[ISH\]) (equivocal).

‣ 2+ by IHC and HER2/CEP17 ratio \<2.0 and/or HER2 copy number \<4.0 by in situ hybridization \[ISH\]) (2+ ISH negative).

‣ Known inhomogeneous HER2-expression in the primary tumor with HER2-positive areas \<10% (inhomogeneous).

• Primary breast cancer planned for neoadjuvant therapy (Stage II-III, T2-4N0-3) or metastatic (M1; at least 80 pts).

• At least one tumor lesion ≥ 10 mm.

• At least one tumor lesion available for biopsy.

• Newly diagnosed or confirmed progression and planned for therapy with trastuzumab emtansine or anti-HER2 targeted therapy(-ies) concomitant with chemotherapy (HER2-positive cohort) or chemotherapy (HER2-negative patients).

• WHO performance status ≤ 2.

• Predicted survival \> 12 weeks.

⁃ Negative pregnancy test in women of childbearing potential (premenopausal or \<12 months of amenorrhea post-menopause and who have not undergone surgical sterilization). Women of childbearing potential must use highly effective method of contraception, i.e combined hormonal contraception, or progestogen-only hormonal contraception, or intrauterine device, or intrauterine hormone-releasing system, or bilateral tubal occlusion, or vasectomized partner, or sexual abstinence