Table 2.

Classification of long mature RNAs.

#	Transcript Mechanism	Genetic Base	RNAs
I	Well characterized	With an annotated or unannotated (including reathrough) gene as a base	Classical mRNAs and noncoding RNAs
			Circular RNAs
		With a fusion gene as a DNA base	Fusion RNAs
II	Unknown	With or without a DNA base?	RNAs with neighboring-genes’ sequences
III	Less known	Without a DNA base	RNAs with sense and antisense sequences
			RNAs with duplicated exons
IV	Unknown	With two genes as bases	Authentic chimeric RNAs

Note: “Transcript mechanism” indicates the regulatory mechanisms for the transcription and posttranscription, including cis-splicing. Readthrough RNAs are considered to be derived from unannotated genes and thus grouped into type I.