design: There are two main ways to obtain mRNA: one is to enrich mRNA with polyA tail by Oligo(dT) magnetic beads, which takes advantage of the structural feature that most mrnas in eukaryotes carry polyA tails. The second is the removal of ribosomal RNA from total RNA, resulting in mRNA. Then, the obtained mRNA was randomly interrupted by divalent cation in NEB Fragmentation Buffer, and the library was constructed according to the NEB general library building method or strand-specific library building method