Circular RNAs (circRNAs) are covalently closed RNAs derived from back-splicing of genes across eukaryotes. Through alternative back-splicing (ABS), a single gene produces multiple circRNAs sharing the same back-splice site. Although many ABS events have recently been discovered, to what extent ABS involves in circRNA biogenesis and how it is regulated in different human tissues still remain elusive. Here, we reported an in-depth analysis of ABS events in 90 human tissue transcriptomes. We observed that ABS occurred for about 84% circRNAs. Interestingly, alternative 5\u27 back-splicing occurs more prevalently than alternative 3\u27 back-splicing, and both of them are tissue-specific, especially enriched in brain tissues. In addition, the patterns of ABS events in different brain regions are similar to each other and are more complex than the patterns in non-brain tissues. Finally, the intron length and abundance of Alu elements positively correlated with ABS event complexity, and the predominant circRNAs had longer flanking introns and more Alu elements than other circRNAs in the same ABS event. Together, our results represent a resource for circRNA research-we expanded the repertoire of ABS events of circRNAs in human tissue transcriptomes and provided insights into the complexity of circRNA biogenesis, expression, and regulation
