Fig. 1

The formation and classification of circRNAs. Canonical splicing produces liner mRNAs (a) while back-splicing generates circRNAs (b). b. CircRNAs were usually categorized into ecircRNAs, ciRNAs and EIciRNAs depending on their components, which were derived from exons and introns, and both of them in pre-mRNAs, respectively. Different ecircRNAs could be generated from one pre-mRNA via alternative splicing. The red segment and yellow segment between Exon 4 and Exon 5 represented a 7-nt GU-rich motif near the 5′ splice site and an 11-nt C-rich motif at the branchpoint site, respectively, which promoted the generation of ciRNAs