library_construction_protocol: Snap29 KO and WT mESCs and differentiated cells were washed by PBS and trypsinized to single cells, and then were committed to RNA extraction The initial RNA for library construction is total RNA, and the mRNA with poly A tail is enriched by Oligo (dT) magnetic beads. Using fragmented mRNA as a template and random oligonucleotides as primers, the first strand of cDNA is synthesized in the M-MuLV reverse transcriptase system, which was followed by second strand cDNA synthesis. The cDNA fragments were added by a single “A” base and subsequently ligated with the adapter. The products were then purified and enriched with PCR amplification. PCR yield was quantified and samples were pooled together to make a single-strand DNA circle (ssDNA circle), generating the final library.