Schematic of splicing of circRNAs. a Generally, linear mRNA is generated through canonical splicing. b Exonic circRNAs (ecircRNAs) are formed through back-splicing from a 5′ splice site (donor site) to a 3′ splice site (acceptor site), containing one exon. c EcircRNAs can also be formed to contain multiple exons, with the intron between two axons removed, thus bringing the 5′ splice site of one exon close to the 3′ splice site of the other exon. d If intron is retained with two exons, an exon-intron circRNA (EIciRNA) is formed. e Circular intronic RNAs (ciRNAs) are the closed-loop structure produced by lariat intron excised from pre-mRNA after pairing with reverse complementary sequences. f tricRNA is generated via a 3′-5′ phosphodiester bond between termini of introns that are removed from pre-tRNA by tRNA splicing enzymes.