• Signed informed consent must be obtained prior to any trial-specific procedure. Note: Candidate patients in France must be affiliated to a Social Security System (or equivalent).

• Male/female patients who are at least 18 years of age on the day of signing informed consent.

• Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2.

• Eligible for any of the following drugs: taxane, carboplatin, trastuzumab, pertuzumab and T-DM1 therapy.

• Histologically confirmed non-metastatic primary invasive adenocarcinoma of the breast untreated and recently diagnosed.

• Stage at presentation: cT1 cN1-2 or cT2-3 cN0-2 as determined by AJCC staging system, 8th edition (specifically in accordance with Anatomic Stage group rules).

• Note: Axillary lymph node status must be assessed by fine needle biopsy or core biopsy. This procedure at screening will be omitted if there is no suspicion for positive axillary lymph node(s) radiographically or if a pathological report of suspicious lymph nodes of the results of a fine needle biopsy or core biopsy is available prior to the screening period.

• Absence of distant metastasis (i.e., cM0).

• Patients with multifocal tumors (more than one mass confined to the same quadrant as primary tumor) are eligible provided at least one focus is sampled and locally confirmed as HER2-positive.

• Patients with multicentric tumors (multiple tumors involving more than one quadrant) are eligible provided all discrete lesions are sampled and locally confirmed as HER2-positive.

• Note: In patients with multifocal or multicentric breast cancer, the largest lesion should be measured to determine T stage and to performe the HER2DX test.

⁃ HER2 positivity defined as either of the following: IHC 3+ or HER2 2+/ ISH positive as per most recent ASCO- CAP guideline according to the local laboratory as determined on the most recently analyzed tissue sample.

⁃ ER/PR status determined local based on pretreatment breast biopsy material according to the most recent ASCO/CAP guidelines.

⁃ Candidates for neoadjuvant treatment.

⁃ Patient agreement to undergo appropriate surgical management, including axillary lymph node surgery and partial or total mastectomy, after completion of neoadjuvant treatment

⁃ Baseline left ventricular ejection fraction (LVEF) ≥ 50% measured by echocardiogram (ECHO) or multiple-gated acquisition (MUGA) scans.

⁃ Availability of pre-treatment tumor tissue sample of FFPE tumor block from primary tumor in the breast for diagnostic HER2DX test. The tumor tissue should be of good quality based on total and viable tumor content and must be evaluated centrally for quality prior to enrollment. Archival tumor tissue or ex professo biopsy are acceptable.

⁃ Adequate hematologic and end-organ function.

⁃ For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods, and agreement to refrain from donating eggs, as defined below:

∙ Women must remain abstinent or use contraceptive methods with a failure rate of \< 1% per year during the treatment period, 6 months after the final dose of doxorubicin, 12 months after the final dose of cyclophosphamide, 6 months after the final dose of paclitaxel, and 7 months after the final dose of trastuzumab, pertuzumab, or T-DM1, whichever occurs last. Women must refrain from donating eggs during this same period.

‣ A woman is of childbearing potential if she is postmenarcheal, has not reached a postmenopausal state (\> 12 continuous months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus). The definition of childbearing potential may be adapted for alignment with local guidelines or requirements.

‣ Examples of contraceptive methods with a failure rate of \< 1% per year include bilateral tubal ligation, male sterilization, copper intrauterine devices, hormonal contraceptives that inhibit ovulation, and hormone-releasing intrauterine devices in women with hormone receptor-negative tumors only; the use of hormonal contraceptives and hormone releasing intrauterine devices are prohibited in women with hormone receptor-positive tumors.

‣ The reliability of sexual abstinence should be evaluated in relation to the duration of the study and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.

⁃ For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and agreement to refrain from donating sperm, as defined below:

∙ With a female partner of childbearing potential who is not pregnant, men who are not surgically sterile must remain abstinent or use a condom plus an additional contraceptive method that together result in a failure rate of \< 1% per year during the treatment period and for 6 months after the final dose of doxorubicin and/or cyclophosphamide, 6 months after the final dose of paclitaxel, and 7 months after the final dose of trastuzumab, pertuzumab, or T-DM1, whichever occurs last. Men must refrain from donating sperm during this same period. Male patients are encouraged to seek advice regarding cryoconservation of sperm prior to commencing study treatment because of the possibility of infertility with CT.

‣ With a pregnant female partner, men must remain abstinent or use a condom during the treatment period and for 6 months after the final dose of doxorubicin and/or cyclophosphamide, 6 months after the final dose of paclitaxel, and 7 months after the final dose of trastuzumab, pertuzumab, or T-DM1, whichever occurs last to avoid exposing the embryo.

‣ The reliability of sexual abstinence should be evaluated in relation to the duration of the study and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.