Fig. 1

Biogenesis of circRNAs. Schematic showing the existing models of circRNA formation. a Intron pairing-driven circularization is accomplished by direct base pairing of the introns flanking complementary sequences or inverted repeats. b RBP-associated pairing-driven circularization is led by the close proximity of circRNA splice sites mediated by complementary base pairing of inverted repeats in the introns flanking the circRNA-forming exons. c Lariat-driven circularization facilitates the formation of an exon-containing lariat precursor starting from an exon-skipping event for efficient circle production. d CiRNA can be generated from intronic lariat precursors that escape the debranching step of canonical linear splicing. e Alternative back-splicing. f Formation of TricRNA. Through these different formation mechanisms, circRNAs can be divided into the following types: EcRNAs (exonic circRNAs), EIciRNAs (exon–intron circRNAs), ciRNAs (circular intronic RNAs) and TricRNAs (tRNA intronic circular RNAs)