What happens at or after transcription: Insights into circRNA biogenesis and function

Circular RNAs (circRNAs) are a large family of noncoding RNAs (ncRNAs) found in metazoans. Systematic studies of circRNAs have just begun. Here, we discuss circRNA biogenesis and functions with a focus on studies indicating great diversification of circRNAs. We highlight the recent identification of a special subtype of circRNAs, called EIciRNAs, and their role in transcriptional regulation. New insights on RNA-RNA interaction and other features associated with circRNA biology are also discussed.