design: The first strand of cDNA was synthesized using fragmented mRNA as template and random oligonucleotides as primers in M-MuLV reverse transcriptase system, followed by RNA strand degradation by RNaseH, and the second strand of cDNA was synthesized using dNTPs under DNA polymerase I system. The purified double-stranded cDNA was end repaired, an A-tail was added, and the sequencing adapter was ligated. The cDNA of about 250-300bp was screened by AMPure XP beads, and the PCR product was purified by AMPure XP beads again