This study identifies and characterizes 7,112 human circular RNAs (circRNAs) and 635 mouse circRNAs using a computational pipeline based on RNA-seq data. The results show that circRNAs are expressed in few cell types at low abundance, but are not more cell-type-specific than similarly expressed mRNAs. Most circRNAs overlap with protein-coding sequences but are not translated, as evidenced by ribosome profiling. The previously known miR-7 sponge, CDR1as, is one of only two circRNAs with more miRNA sites than expected by chance. The study also finds that circRNAs are not more conserved than their neighboring linear exons, suggesting that most circRNAs may not have a biological function. The results suggest that circRNAs are likely to be side products of imperfect pre-mRNA splicing rather than functional regulatory molecules. The study highlights the importance of circRNAs in post-transcriptional regulation and their potential as miRNA sponges. The findings provide a new framework for future research on circRNAs and raise questions about their biological functions.This study identifies and characterizes 7,112 human circular RNAs (circRNAs) and 635 mouse circRNAs using a computational pipeline based on RNA-seq data. The results show that circRNAs are expressed in few cell types at low abundance, but are not more cell-type-specific than similarly expressed mRNAs. Most circRNAs overlap with protein-coding sequences but are not translated, as evidenced by ribosome profiling. The previously known miR-7 sponge, CDR1as, is one of only two circRNAs with more miRNA sites than expected by chance. The study also finds that circRNAs are not more conserved than their neighboring linear exons, suggesting that most circRNAs may not have a biological function. The results suggest that circRNAs are likely to be side products of imperfect pre-mRNA splicing rather than functional regulatory molecules. The study highlights the importance of circRNAs in post-transcriptional regulation and their potential as miRNA sponges. The findings provide a new framework for future research on circRNAs and raise questions about their biological functions.