Inclusion Criteria:

          -  Female gender;

          -  Age ≥18 years;

          -  Eastern Cooperative Oncology Group performance status of 0-1;

          -  Histologically confirmed invasive breast cancer;

          -  Primary tumor greater than or equal to 1 cm diameter, as measured by clinical
             examination and mammography or ultrasound;

          -  Any N;

          -  No evidence of metastasis (M0) (isolated supra-clavicular node involvement allowed);

          -  HER2 low or equivocal status in the invasive component of the primary tumor (confirmed
             by a central certified laboratory prior to study entry)

               -  HER2 low expression: 1+/2+ by immunohistochemistry and/or HER2/CEP17 ratio <2.0
                  with HER2 copy number <6.0 signals/cell

               -  HER2 equivocal expression: HER2 copy number ≥4.0 and <6.0 signals/cell;

          -  Hematopoietic status:

        Absolute neutrophil count ≥ 1.0 x 10^9/L, Platelet count ≥ 100 x 10^9/L, Hemoglobin at
        least 9 g/dL;

        • Hepatic status: Serum total bilirubin ≤1 x upper limit of normal (ULN; In the case of
        known Gilbert's syndrome, a higher serum total bilirubin [< 1.5 x ULN] is allowed),
        Aspartate aminotransferase and alanine aminotransferase ≤1.5 x ULN, Alkaline phosphatase ≤
        1.5 x ULN;

        • Renal status: Creatinine ≤1.5 mg/dL;

          -  International Normalized Ratio ≤1.5 x ULN;

          -  Baseline left ventricular ejection fraction ≥50%, as measured by echocardiography or
             multigated acquisition scan;

          -  Negative serum or urine β-human chorionic gonadotropin pregnancy test within 7 days
             prior study entry for patients of childbearing potential. Women of childbearing
             potential must use effective contraception (barrier method [condoms, diaphragm] in
             conjunction with spermicidal jelly, or total abstinence. Oral, injectable, and
             implantable hormonal contraceptives are not allowed) for the duration of the study and
             for at least 7 months after the last dose of study treatment;

          -  Signed informed consent form (ICF);

          -  Patient accepts to make available tumor samples for submission to central laboratory
             to conduct translational studies as part of this protocol.

        Exclusion Criteria:

          -  Previous (less than 5 years) or current history of malignant neoplasms, except for
             curatively treated basal and squamous cell carcinoma of the skin and carcinoma in situ
             of the cervix;

          -  Patients with a prior malignancy diagnosed more than 5 years prior to study entry;

          -  Preexisting peripheral neuropathy ≥ grade 2;

          -  Known history of uncontrolled or symptomatic angina, clinically significant
             arrhythmias, congestive heart failure, transmural myocardial infarction, uncontrolled
             hypertension (≥180/110), unstable diabetes mellitus, dyspnea at rest, or chronic
             oxygen therapy;

          -  Concurrent disease or condition that would make the patient inappropriate for study
             participation or any serious medical disorder that would interfere with the patient's
             safety;

          -  Unresolved or unstable serious adverse events from prior administration of another
             investigational drug;

          -  Dementia, altered mental status, or any psychiatric condition that would prevent the
             understanding or rendering of ICF;

          -  Concurrent neoadjuvant cancer therapy (chemotherapy, radiation therapy, immunotherapy,
             or biologic therapy other than the trial therapy);

          -  Concurrent treatment with an investigational agent or participation in another
             therapeutic clinical trial;

          -  Known immediate or delayed hypersensitivity reaction or idiosyncrasy to drugs
             chemically related to trastuzumab emtansine or its components;

          -  Ejection fraction <55% or below the lower limit of the institutional normal range;

          -  Pregnant or lactating women;

          -  Concomitant use of cytochrome P450 3A4 inhibitors or inducers;

          -  Other concurrent severe and/or uncontrolled concomitant medical conditions (e.g.,
             active or uncontrolled infection, uncontrolled diabetes) that could cause unacceptable
             safety risks or compromise compliance with the protocol;

          -  Active infection requiring intravenous or oral antibiotics;

          -  Patients unwilling or unable to comply with the protocol.