Inclusion Criteria:

          -  Eastern Cooperative Oncology Group (ECOG) performance 0-2

          -  Life expectancy of greater than 12 weeks

          -  Histologically or cytologically proven metastatic breast cancer (metastases can be
             proven with imaging results in certain circumstances provided that the initial tumor
             was demonstrated histologically)

          -  Stage IV HER2/Neu positive breast cancer patients who failed previous anti-HER2
             targeted therapies

          -  HER2 positivity as defined by American Society of Clinical Oncology (ASCO)/College of
             American Pathologists (CAP) guidelines

          -  If HER2 negative by immunohistochemistry (IHC) or fluorescence in situ hybridization
             (FISH), but activating somatic mutations of HER2 gene identified through genomic
             sequencing including but not limited to the following (Clinical Laboratory Improvement
             Act [CLIA] certified lab test): missense substitutions (G309A, G309E, S310F, S310Y,
             S653C, V659E, R678Q, V697L, T733I, L755S, L755P, E757A, D769H, D769Y, D769N, G776V,
             G776C, V777L, L841V, V842I, R849W, L869R, R896C); insertions/deletions
             (A775_G776insYVMA aka Y772_A755dup, G776VinsC, G776AinsVGC, G776 insertions,
             G778_S779insCPG, P780_781insGSP aka G778_P780dup, L755_T759del) and/or HER3 activating
             mutations; there is no limitation on the number of prior lines of systemic therapy or
             HER2-targeted therapies (prior neratinib not allowed)

          -  Both measurable as well as non-measurable disease will be allowed

          -  Hemoglobin >= 9 g/dL (after transfusion, if necessary) within 4 weeks of
             pre-registration

          -  Total bilirubin within normal institutional limits within 4 weeks of pre-registration

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase
             [SGOT])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SGPT])
             =< 2.5 x institutional upper limit of normal within 4 weeks of pre-registration

          -  Creatinine clearance >= 30 mL/min as calculated by Cockcroft-Gault formula within 4
             weeks of pre-registration

          -  Baseline left ventricular ejection fraction LVEF >= 50% as evaluated by echocardiogram
             (ECHO) or multiple-gated acquisition scan (MUGA)

          -  All grade >= 2 toxicities other than alopecia from prior therapy have resolved by the
             time of study commencement

          -  Patient must have completed radiation therapy with adequate recovery of bone marrow
             and organ functions, before starting neratinib

          -  Patient with stable or treated brain metastases are eligible; asymptomatic patients
             with metastatic brain disease who have been on a stable dose of corticosteroids for
             treatment of brain metastases for at least 14 days are eligible to participate in the
             study

          -  Provide written, informed consent to participate in the study and follow the study
             procedures

        Exclusion Criteria:

          -  Prior treatment with neratinib

          -  Concurrent usage of other investigational agents, chemotherapy, or hormone therapy;
             prior chemotherapy, hormonal therapy, targeted therapy, and investigational agents are
             allowed but all toxicities grade >= 2 must have resolved by the time of study
             commencement (except alopecia)

          -  Any major surgery =< 28 days prior to the initiation of investigational products

          -  Received chemotherapy or biologic therapy =< 3 weeks prior to the start of neratinib

          -  Active uncontrolled cardiac disease, including cardiomyopathy, congestive heart
             failure (New York Heart Association functional classification of >= 2, including
             individuals who currently use digitalis specifically for congestive heart failure),
             unstable angina, myocardial infarction within 12 month of enrollment or ventricular
             arrhythmia

          -  Concurrent use of digoxin due to cardiac disease; corrected QT (QTc) interval >= 450
             milliseconds in men and >= 470 milliseconds in women within 2 weeks of registration or
             known history of QTc prolongation or Torsades de Pointes

          -  Inability to take oral medication

          -  Other malignancy within the past 3 years with the exception of: a) adequately treated
             basal cell carcinoma, squamous cell skin cancer, or thyroid cancer; b) cervix or vulva
             carcinoma in situ; c) cancer considered cured by surgical resection or unlikely to
             impact survival during the duration of the study, such as localized transitional cell
             carcinoma of the bladder, or benign tumors of the adrenal or pancreas

          -  Significant chronic gastrointestinal disorder with diarrhea as a major symptom (e.g.,
             Crohn?s disease, malabsorption, or grade >= 2 National Cancer Institute [NCI] Common
             Terminology Criteria for Adverse Events [CTCAE] v.4.0 diarrhea of any etiology at
             baseline)

          -  Known clinically active infection with hepatitis B or hepatitis C virus

          -  Evidence of significant medical illness, abnormal laboratory finding or psychiatric
             illness/social situations that would, in the investigator?s judgment, makes the
             patient inappropriate for this study