Fig. 1.

Formation of three types of circRNAs. a Exonic circular RNA (ecircRNA) is formed through back-splicing of the 5’splice site (splice donor site) to a 3’splice site (splice acceptor site); (b) The intron 1 is removed and bring the 5′ splice site of Exon 2 close to 3′ splice site of Exon 1, to form a ecircRNA that contains multiple exons. Exons can also skip splicing, exon 1 can also link with exon 3; (c) Circular intronic RNA (ciRNA) are derived from intron lariats that escape the normal intron debranching and degradation. Reverse complementary sequences of lariat intron excised from pre-mRNA can pair to produce close loop structure termed as ciRNA; (d) Exon–intron circRNAs (EIciRNAs) are circularized with introns‘retained’between the exons. Intron 3 retaines with Exon 3 and Exon 4 to form an EIciRNAs