circRNA is an emerging class of widespread, abundant, and evolutionarily conserved noncoding RNA, and generated by non-sequential back splicing of exons, introns, or both. In this century, the vital functions of circRNAs in eukaryotes have increasingly come to light. Recently, with the development of RNA sequencing and microarray, thousands of types of circRNAs have been found to be endogenous in mammalian cells. Various physiological functions are associated with circRNAs, such as acting as miRNA sponges, RNA banding protein sponges, protein translation templates, and regulation of gene expression. In addition, circRNAs are abundant in eukaryotes and are involved in many diseases, including different cancers and autoimmune diseases.
