Thousands of loci in the human and mouse genomes give rise to circular RNA 
transcripts; at many of these loci, the predominant RNA isoform is a circle. 
Using an improved computational approach for circular RNA identification, we 
found widespread circular RNA expression in Drosophila melanogaster and estimate 
that in humans, circular RNA may account for 1% as many molecules as poly(A) 
RNA. Analysis of data from the ENCODE consortium revealed that the repertoire of 
genes expressing circular RNA, the ratio of circular to linear transcripts for 
each gene, and even the pattern of splice isoforms of circular RNAs from each 
gene were cell-type specific. These results suggest that biogenesis of circular 
RNA is an integral, conserved, and regulated feature of the gene expression 
program.