Fig. 1

The representative summarized models of circRNA formation. a Three typical circularization forms. Intron pairing-driven circularization is mediated by cis-acting regulatory elements that include reverse complementary sequences (Alu repeats), contributing direct base pairing of flanking introns. Lariat driven circularization, lariats formation of pair intron make exon skipping through back splicing and lead to the formation of ecircRNA or EIciRNA. Trans-acting factors, such as RNA-binding proteins (RBPs) or several splicing factors that bind to specific sequence motifs of flanking introns, promote circRNA biogenesis. GU rich and C rich elements contribute to generating intronic RNA following the canonical splicing. b Intergenic circRNAs are generated from the distant regions between genes. c The formation of circular LINC-PINT exon