Circular RNAs (circRNAs) are now recognized as large species of transcripts in eukaryotic
cells. From model organisms such as C. elegans, Drosophila, mice to human beings, thousands of
circRNAs formed from back-splicing of exons have been identified. The known complexity of
transcriptome has been greatly expanded upon the discovery of these RNAs. Studies about the
biogenesis and physiological functions have yielded substantial knowledge for the circRNAs, and they
are now more likely to be viewed as regulatory elements coded by the genome rather than unavoidable
noise of gene expression. Certain human diseases may also relate to circRNAs. These circRNAs
show diversifications in features such as sequence composition and cellular localization, and thus we propose that they
may be divided into subtypes such as cytoplasmic circRNAs, nuclear circRNAs, and exon-intron circRNAs (EIciRNAs).
Here we summarize and discuss knowns and unknowns for these RNAs, and we need to keep in mind that the whole field
is still at the beginning of exciting explorations.
Keywords: Biogenesis, circRNA, EIciRNA, microRNA, microRNA sponge, ncRNA, Transcription.
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