Due to the covalently close structure and without 3 poly(A)tails,circular RNAs from back-spliced exons(circRNAs)were failed to be analyzed in most of polyadenylated transcriptome analyses.By taking advantage of the nonpolyadenylated RNA-seq(p(A)– RNA-seq)and a specific computational pipeline for back-splicing junction prediction(CIRCexplorer),thousands of circRNAs have been identified among different cell lines/species,and their biogenesis has been shown to be associated with and enhanced by RNA pairs of orientation-opposite complementary sequences in flanking introns.Importantly,circRNA formation is regulated by competition between different RNA pairs across flanking introns or within individual introns1.