Fig. 1

Three paths of circRNAs formation. Legend: There are four commonly acknowledged paths in the formation of circRNAs. a Spliceosome-Dependent Circulation Path: the processing of the back-splicing reaction requires the sequential assembly of snRNPs on the pre-mRNA to catalyse circulation. b Intron-Pairing-Driven Circularization Path: circRNA are formed depend on an intronic reverse complementary motif containing a GU-rich element and a C-rich element in two sides to promote circularization. c Lariat-Driven Circularization Path: formation of circRNA is promoted by a lariat structure; the ALU complementary flanking elements repeated in intronic regions compete with canonical linear-RNA splicing and accelerate the circularization by reverse complementary matches. d Protein Factors Associated Circulation Path: some RBPs can bind to specific targets in introns and bring the donor-acceptor sequences in proximity to each other thus trigger several circularization processes. At the mean time, RBPs also stabilize the splicing motifs and inhibit canonical linear splicing