Table 1.

Different types of circRNAs

Categories	Composition characteristics	Main positioning	The main function	Ref.
CiRNAs	Intron splicing	Nucleus	Participate in regulating the expression of its parental genes	[20]
EcircRNAs	Exon splicing	Cytoplasm	As a sponge of miRNA or interact with RBPs	[34]
EIciRNAs	Contains introns and exons	Nucleus	Promote the transcription of its host gene by interacting with U1 small nuclear ribonucleoprotein (snRNP)	[22]
f−circRNAs	Linear fusion transcript derived from genome rearrangement (chromosomal translocation)	Cytoplasm and nucleus	Contribute to cell transformation, promote cell viability and drug resistance after treatment, and have tumor-promoting properties in in vivo models	[23]
rt− circRNAs	Coding exons of two adjacent and similarly oriented genes (read-through transcription)	Cytoplasm	To be elucidated, it may be a mechanism of gene regulation in a specific environment; it may be a mechanism of protein complex evolution	[26]
mecciRNAs	Mitochondrial Genome	Mitochondria inside and outside	As a molecular chaperone, it helps to fold nuclear-encoded proteins and facilitate their entry into the mitochondria.	[45]