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Inclusion Criteria:

* Aged \>/= 18 years
* Prospective centrally assessed HER2-positive disease (i.e., immunohistochemistry \[IHC\] 3+ and/or gene amplified \[HER2 to Chromosome 17 \[CEP 17\] ratio \>/= 2\]) by in situ hybridization (ISH) through use of archival paraffin-embedded tumor tissue
* Histologically or cytologically confirmed invasive breast cancer (BC): incurable, unresectable LABC previously treated with multimodality therapy or MBC
* Prior treatment for BC in the adjuvant, unresectable locally advanced or metastatic setting must include both: a taxane, alone or in combination with another agent, and trastuzumab, alone or in combination with another agent in the adjuvant, unresectable locally advanced or metastatic setting
* Documented progression of incurable, unresectable LABC or MBC, defined by the investigator: progression must occur during or after most recent treatment for LABC or MBC or within 6 months after completing adjuvant therapy
* Measurable and/or non-measurable disease, according the Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 definition: CNS-only disease excluded
* Left ventricular ejection fraction (LVEF) \>/=50% by either echocardiogram or multiple-gated acquisition
* Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1
* Adequate organ function evidenced by laboratory results within 30 days prior to randomization
* For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use non-hormonal contraceptive methods that result in a failure rate of \< 1% per year during the treatment period and for at least 7 months after the last dose of study drug
* For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures (use a condom plus an additional contraceptive method that together result in a failure rate of \< 1% per year) and agreement to refrain from donating sperm during the treatment period and for at least 7 months after the last dose of study drug

Exclusion Criteria:

* History of treatment with trastuzumab emtansine
* Prior treatment with lapatinib or capecitabine
* Peripheral neuropathy of Grade \>/= 3 per National Cancer Institute Common Terminology Criteria for Adverse Events Version 4.0 (NCI CTCAE v4.0)
* History of other malignancy within the previous 5 years, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage 1 uterine cancer, synchronous or previously diagnosed HER2-positive BC, or cancers with a similar curative outcome as those mentioned above
* History of receiving any anti-cancer drug/biologic or investigational treatment within 21 days prior to randomization, except hormone therapy which can be given up to 7 days prior to randomization
* History of radiation therapy within 14 days before randomization
* Brain metastases that are untreated, symptomatic, progressive, or require therapy such as radiation, surgery or corticosteroid therapy to control symptoms from brain metastases within 30 days before randomization
* History of exposure to cumulative doses of anthracyclines: Doxorubicin \> 500 milligrams per square meter (mg/m\^2), Epirubucin \> 720 mg/m\^2, Mitoxantrone \> 120 mg/m\^2
* Current severe, uncontrolled systemic disease (e.g., clinically significant cardiovascular, pulmonary, or metabolic disease)
* Pregnancy or lactation
* Currently known active infection with HIV, hepatitis B virus (HBV), or hepatitis C virus (HCV)
* Presence of conditions that could affect gastrointestinal absorption
* History of intolerance (including Grade 3 or 4 infusion reaction) or hypersensitivity to trastuzumab or murine proteins
* Known hypersensitivity to 5-fluorouracil or known dihydropyrimidine dehydrogenase deficiency
* Current treatment with sorivudine or its chemically related analogs

• Beijing, China
• Beijing, China
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