Advances in Bioscience and Biotechnology, 2015, 6, 433-441 Published Online June 2015 in SciRes. http://www.scirp.org/journal/abb http://dx.doi.org/10.4236/abb.2015.66043 How to cite this paper: Andreeva, K. and Cooper, N.G.F. (2015) Circular RNAs: New Players in Gene Regulation. Advances in Bioscience and Biotechnology, 6, 433-441. http://dx.doi.org/10.4236/abb.2015.66043 Circular RNAs: New Players in Gene Regulation Kalina Andreeva, Nigel G. F. Cooper * Department of Anatomical Sciences and Neurobiology, University of Louisville School of Medicine, Louisville, USA Email: * nigelcooper@louisville.edu Received 27 May 2015; accepted 26 June 2015; published 29 June 2015 Copyright © 2015 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Abstract The existence of circular RNAs (circRNAs) was demonstrated over 30 years ago. They did not gain much interest at the time because they appeared to be relatively rare when compared to the ab- undance of the canonical linear RNAs. However, more recent evidence suggests that circRNAs are abundant in cells and tissues and possess intriguing biological properties. These recent develop- ments have renewed our interest in this novel class of molecules. This report will provide an over- view of circRNAs, discuss how they may modify our understanding of gene regulation and indicate their most likely relevance to health. The circRNAs from viruses, bacteria and archaea are not in the scope of this report, and we focused this review on circRNAs in eukaryotes. Keywords Circular RNA, microRNA Sponges, Gene Regulation 1. Introduction Circular RNAs (circRNAs) are closed RNA loops that lack the terminating 5'-cap and 3'-polyadenylated tail- structures of linear RNAs. They are ubiquitous and have been detected in various speciеs across kingdoms [1]-[7]. CircRNAs may consist of transcribed exons with or without introns that are scrambled (e.g. a down- stream sequence is spliced to an upstream one) [8]. Due to their closed loop form, they cannot be mapped di- rectly to the genome, which is probably one of the reasons for the late discovery of these relatively new RNA species. They appear to be ancient, conserved regulators of eukaryotic gene expression [9] and were initially de- tected in ribosomal depleted RNA-seq data [2]. Due to the lack of free ends circRNAs are resistant to exonuc- leases and have reduced rates of turnover [10]. Their lengths range between a few hundred to thousands of nuc- * Corresponding author.