2.3 DNA extraction
The tissue samples were ground to powder with liquid nitrogen and diluted with three volumes of phosphate‐buffered saline (PBS). The samples (1 ml) were centrifuged at 1,2000×g for 10 min at 4°C and the supernatants were transferred to a 1.5 ml tube. Nucleic acids from tissues and serum samples were extracted by using 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.