Fig. 1

Biogenesis and classification of circRNAs. A Lariat-driven circularization. The 5' end of exon D combines with the 3' end of exon A to form a lariat structure. Then, one or both introns of the lariat structure are excised, resulting in the formation of EIciRNA or EcircRNA, respectively. B, C Intron-pairing-driven circularization. The Alu complementarity (B) or RBPs (C) located on the introns bring one another closer and thus promote the formation of EIciRNA or EcircRNA. D The 3′ and 5′ ends of the intron between exon A and exon B bind close to one another and cyclize, resulting in the formation of ciRNA. E Pre-tRNA is cut by the tRNA splicing endonuclease complex (TSEN) and the ends are connected to form tRNA and tricRNA