2.1. Patients and sample collection

GC tissues and paired adjacent normal tissues were obtained from 114 surgical patients between April 2014 and December 2017 at Ningbo First Hospital, China. The tissue samples were soaked immediately in RNA‐fixer Reagent (Bioteke, Beijing, China) after surgical removal from patients’ bodies and were stored at −80°C until analysis. Paired adjacent normal tissues were taken 5 cm from the cancer edge of GC and contained no obvious tumor cells, as assessed by two professional pathologists. No radiotherapy or chemotherapy was given to these patients before surgery. A total of 48 patients’ plasma samples were collected from peripheral veins before any treatment was applied. Based on age‐ and gender‐matching standards, 48 healthy people at Ningbo First Hospital, China, also had fresh plasma samples collected. All blood samples were treated with ethylenediaminetetraacetic acid (EDTA) as the anticoagulant and stored at −80°C until analysis as previously described.

Tumors were staged according to the Tumor‐Node‐Metastasis (TNM) staging system of the International Union Against Cancer. Histological grade was assessed according to the National Comprehensive Cancer Network (NCCN) Clinical Practice Guideline of Oncology (V.1.2011).

The Human Research Ethics Committee from Ningbo First Hospital approved all aspects of this study. All participants voluntarily joined this research with written informed consent before clinical trial in the study.