design: First, the mRNA with polyA tail is enriched with magnetic beads with OligodT. Then use random N6 primers for reverse transcription, and then synthesize cDNA double strand to form double strand DNA. Then the ends of the synthesized double-stranded DNA were flattened and phosphorylated at the 5 'end, forming a sticky end protruding an ""A"" at the 3' end, and then connecting a bubble-like joint with a protruding ""T"" at the 3 'end. The ligated products were amplified by PCR with specific primers. The PCR product was thermally denatured into a single strand, and then the single strand DNA was cyclized with a bridge primer to obtain a single strand cyclic DNA library.