Circular RNAs are found in a wide range of organisms and it has been proposed 
that they perform disparate functions. However, how RNA circularization is 
connected to alternative splicing remains largely unexplored. Here, we 
stimulated primary human endothelial cells with tumor necrosis factor α or tumor 
growth factor β, purified RNA, generated >2.4 billion RNA-seq reads, and used a 
custom pipeline to characterize circular RNAs derived from coding exons. We find 
that circularization of exons is widespread and correlates with exon skipping, a 
feature that adds considerably to the regulatory complexity of the human 
transcriptome.