Inclusion Criteria: - Cytological or histological confirmation of locally advanced/metastatic or recurrent epithelial ovarian cancer, fallopian tube cancer, primary peritoneal cancer or endometrial cancer; OR, cytological or histological confirmation of locally advanced /metastatic Her2 non-overexpressing breast cancer - Eighteen years of age or above - Candidates for chemotherapy - Able to understand and sign an informed consent (or have a legal representative who is able to do so) - Measurable disease according to RECIST v1.1 - ECOG Performance Score (PS) of ≤ 2 - Willing to abstain from sexual intercourse or to use an effective form of contraception during the study and for 90 days following the last dose of MM-121 Exclusion Criteria: - Prior radiation therapy to >25% of bone marrow-bearing areas - Evidence of any other active malignancy - Active infection or fever> 38.5°C during screening visits or on the first scheduled day of dosing - Symptomatic CNS disease - Known hypersensitivity to any of the components of MM-121 or who have had hypersensitivity reactions to fully human monoclonal antibodies - Received treatment, within 30 days prior to the first scheduled day of dosing, with any investigational agents that have not received regulatory approval for any indication or disease state - Pregnant or breast feeding