CircRNAs are a class of newly discovered RNAs extensively expressed in eukaryotic cells (Jeck et al., 2013). Compared with conventional linear RNAs (containing 5′ and 3′ ends), circRNAs are covalently closed RNA molecules. It was recognized as a class of functional RNA that differs from miRNA, piRNA and lncRNA due to its character of circularity (Xu, Wu, Han, Zhao, & Song, 2017). CircRNAs were first found in RNA viruses as a viroid as early as in 1976 (Sanger, Klotz, Riesner, Gross, & Kleinschmidt, 1976). And then in 1986, it was also founded that the HDV, an animal virus, has a circRNA genome, which were considered to be a characteristic of viroids (Kos, Dijkema, Arnberg, van der Meide, & Schellekens, 1986). Due to limitation of technology at that time, circRNAs were less reported and were once considered as by-products of mis-splicing or regarded as a meaningless abnormally cut RNA, which never got any attention by some researchers because traditional molecular techniques employed in RNA analysis were not able to detect circRNAs because they did not have cap of 5′ end and tail of 3′ end (Jeck & Sharpless, 2014). It was not until 1991 that endogenous circRNAs were demonstrated in human cells, which originated from exons (Nigro et al., 1991). Since then, with advancement of bioinformatics analysis and RNA sequencing technologies, circRNA has attracted more and more attention of many scientists because it has been reported to be ubiquitously present in eukaryotic organisms and have longer half-lives than linear RNAs (Memczak et al., 2013;