Fig. 2.

Summary of circRNA biology. Both circRNAs and mRNAs are originated from pre-mRNAs transcribed from genomic DNA which are regulated by transcriptional regulators. Distinguished from mRNA maturation (5′-capping, 3′-polyadenylation and introns removing), circRNA maturation go through various back-splicing processes which competes with the traditional mRNA splicing. CircRNAs matured in the nucleus may stay in the nucleus (EIcRNAs and ciRNAs with intron elements) or export to the cytoplasm (EcRNAs without introns). CircRNAs stay in the nucleus may participate in nuclear processes such as transcriptional regulation. CircRNAs export to the cytoplasm are involved in cytoplasmic processes such as miRNA sponging, RBP binding and translation. Some circRNAs are also enriched and stable in exosomes and secreted to extracellular components. All the circRNAs will eventually be degraded via a variety of mechanisms