design: The first strand of cDNA was synthesized in the M-MuLV reverse transcriptase system using fragmented mRNA as a template and random oligonucleotides as primers, followed by degradation of the RNA strand with RNaseH and synthesis of the second strand of cDNA with dNTPs in DNA polymerase I system. After purified double-stranded cDNA was end-repaired, A-tailed, and connected to sequencing junction, cDNA around 250-300bp was screened by AMPure XP beads, PCR amplification was performed and PCR products were purified again using AMPure XP beads to finally obtain the library.