2.3 DNA extraction
The tissue samples were ground to powder with liquid nitrogen and diluted with three volumes of phosphate‐buffered saline. The samples (1 ml) were centrifuged at 12,000×g for 10 min at 4°C and the supernatants were transferred into a 1.5 ml tube. Nucleic acids from tissues and serum samples were extracted by using an EasyPure®Viral DNA/RNA Kit (TransGen Biotech, Beijing, China) according to the manufacturer’s instructions. The extracted genomic DNA was stored at -20°C before use.