Fig. 1

The proposed models of circRNA formation. a Direct cyclization of lariat introns. Canonical linear splicing generates a lariat structure. The 3′ downstream of the lariat intron is trimmed to form a circular intronic RNA (ciRNA). b Cyclization driven by lariat mechanism. The exon-skipping event during alternative splicing promotes the 3′ splice site (3′SS) of the exon to covalently splice to the 5′ splice site (5′SS). c Cyclization mediated by intron pairing. Intron pairing brings the appropriate splice signals within proximity of each other, which promotes cyclization. d Cyclization mediated by RNA-binding proteins (RBPs). RBPs bring the appropriate splice signals within proximity of each other, which promotes cyclization. e Cyclization driven by tRNA splicing. f Cyclization driven by rRNA splicing