Fig. 1

Schematic illustration of three biosynthesis pathways of circular RNA. Messenger RNA can be spliced through canonical splicing to produced mature mRNA and non-canonical splicing to produce circular RNAs. There are three back splicing models in non-canonical splicing that produced three types of circular RNAs. a The intron-pairing circularisation that produces all three types of circular RNAs: circular intronic RNAs (ciRNAs), exon–intron circular RNAs (EIciRNAs), and exonic circular RNAs (ecircRNAs). b The lariat driven circularisation that produces two types of circular RNAs: EIicRNAs, ecircRNAs. c The RNA binding protein mediated circularisation that produces two types of circular RNAs: EIicRNAs, ecircRNAs. Of which, ciRNAs are derived from introns and localise within the nucleus; EIciRNAs are derived from both exons and introns and localise within the nucleus; ecircRNAs are derived from exons and localise within the cytoplasm [150]