Inclusion Criteria:

          -  Histologically or cytologically confirmed invasive carcinoma of the breast

          -  Treatment with at least two prior systemic therapies for advanced (unresectable
             locoregional or metastatic) disease

          -  Evidence of HER2 oncoprotein expression at the 2+ level by central laboratory.
             Patients whose tumors exhibit 2+ staining by IHC are eligible for the study.

          -  Patients whose tumors score 1+ by conventional IHC, are non-amplified by FISH testing,
             and whose tumors score > or = 10.5 by HERmark® testing, are eligible for the study.

          -  Evidence of lack of HER2 oncogene amplification as determined by FISH testing by
             central laboratory

          -  Performance Status of 0 or 1

          -  Life expectancy at least 6 months

          -  Measurable disease (by RECIST 1.1)

          -  Acceptable laboratory parameters and organ reserve

          -  Baseline left ventricular ejection fraction > or = 50%

          -  Anti-cancer therapy (including conventional cytotoxic chemotherapy and/or biological
             therapy) and radiotherapy must be completed and any associated toxicities resolved to
             </= Grade 1 levels or baseline levels and at least 2 weeks must have elapsed before
             enrollment. Treatment with monoclonal antibodies must be completed at least 14 days
             before entry. Must have completed immunosuppressive medications or vaccinations before
             enrollment.

          -  Patients who are estrogen receptor+ and/or progesterone receptor+ and who are
             receiving anti-hormone therapy for at least three months may continue to receive such
             therapy during the course of the trial

          -  Eighteen (18) years of age or older

        Exclusion Criteria:

          -  Major surgery or trauma within 4 weeks

          -  Known hypersensitivity to murine or recombinant proteins, polysorbate 80, or any
             excipient contained in the margetuximab drug formulation

          -  Second primary malignancy that has not been in remission for more than 3 years

          -  History of active viral, bacterial, or systemic fungal infection requiring parenteral
             treatment within 14 days

          -  History within 3 months of deep vein thrombosis, pulmonary embolism, or stroke

          -  Symptomatic or untreated central nervous system (CNS) metastatic disease. Patients
             with previously treated CNS metastatic disease which has been stable for at least 56
             days are eligible

          -  Requirement, at time of study entry, for concurrent steroids > 10 mg/day of oral
             prednisone or the equivalent, except steroid inhaler, nasal spray, or ophthalmic
             solution

          -  Serious medical condition that would impair the ability to receive or tolerate
             margetuximab; dementia or altered mental status that would preclude provision of
             informed consent

          -  Uncontrolled hypertension, heart disease including history of congestive heart
             failure, history of myocardial infarction, angina pectoris requiring medication,
             clinically significant valvular heart disease, high risk arrhythmias, or disease
             corresponding to New York Heart Association class III or IV.

          -  Significant pulmonary compromise

          -  Have previously been exposed to MGAH22 in this or any other trial