Table 1.

CircRNAs classification.

Category	Abbreviations	Cellular compartment	Mechanism of biogenesis	Ref
Exon-derived circRNAs	ecircRNA	Cytoplasm	1–5 exon circularization	[18, 20, 21]
Exon–intron circRNAs	EIciRNA	Nucleus	Intronic excision failure followed by intronic retention in the nascent circRNAs	[68]
Intronic circRNAs	ciRNA	Nucleus	Intron lariats escaping the usual intron debranching and following degradation	[56, 77, 78]
Intergenic circRNAs	/	/	From intergenic genomic portions and formed by two intronic circRNA fragments flanked by GT-AG (CT-AC) splicing signals acting as the splice donor and acceptor of the circular junction in the process of forming an integrated circRNA	[18, 80]
Read-through circRNAs	rt-circRNA	Cytoplasm	Read-through of polymerase that includes exons from adjacent genes	[49, 81]
Fusion-circRNAs	f-circRNAs	Cytoplasm/ Nucleus	Cancer-associated chromosomal translocation	[83, 84]