To identify the correlation of HER2 expression in tissue and peripheral CTC. to identify the HER2 expression in CTCs with clinical prognosis in advanced/metastatic gastric cancer.

Confirm the presence of CTCs are sensitive for monitoring response to chemotherapy.

Inclusion Criteria:

• Having signed informed consent
• Age≥ 18 years old
• HER2 overexpression confirmed by IHC or ISH (IHC 3+,or IHC2+/ISH+)
• Histologically confirmed gastric adenocarcinoma
• Unresectable recurrent or metastatic disease
• Previous neo-adjuvant or adjuvant treatment for gastric cancer, if applicable, more than 6 months
• Measurable disease according to the RECIST criteria
• Karnofsky performance status ≥70
• Life expectancy of ≥3 month
• No prior radiotherapy except radiotherapy at non-target lesion of the study more than 4 weeks
• ALT and AST<2.5 times ULN (≤5 times ULN in patients with liver metastases)
• Serum albumin level ≥3.0g/dL
• Serum AKP < 2.5 times ULN
• Serum creatinine <ULN, and CCr < 60ml/min
• Bilirubin level < 1.5 ULN
• WBC>3,000/mm3, absolute neutrophil count ≥2000/mm3, platelet>100,000/mm3, Hb>9g/dl

Exclusion Criteria:

• Previous systemic therapy for metastatic gastric cancer
• Surgery (excluding diagnostic biopsy) within 4 weeks prior to study entry Contraindications of nuclear magnetic resonance image such as fitment of cardiac pacemaker , nerve stimulator, or aneurysm clip, and metallic foreign body in eye ball and so on.
• Allergic constitution or allergic history to protium biologic product or any investigating agents.
• Severe heart disease or such history as recorded congestive heart failure, uncontrolled cardiac arrhythmia, angina pectoris needing medication, cardiac valve disease, severe abnormal ECG findings, cardiac infarction , or retractable hypertension.
• Pregnancy or lactation period
• Other previous malignancy within 5 year, except non-melanoma skin cancer
• Legal incapacity