Protein-coding and noncoding genes in eukaryotes are typically expressed as 
linear messenger RNAs, with exons arranged colinearly to their genomic order. 
Recent advances in sequencing and in mapping RNA reads to reference genomes have 
revealed that thousands of genes express also covalently closed circular RNAs. 
Many of these circRNAs are stable and contain exons, but are not translated into 
proteins. Here, we review the emerging understanding that both, circRNAs 
produced by co- and posttranscriptional head-to-tail "backsplicing" of a 
downstream splice donor to a more upstream splice acceptor, as well as circRNAs 
generated from intronic lariats during colinear splicing, may exhibit 
physiologically relevant regulatory functions in eukaryotes. We describe how 
circRNAs impact gene expression of their host gene locus by affecting 
transcriptional initiation and elongation or splicing, and how they partake in 
controlling the function of other molecules, for example by interacting with 
microRNAs and proteins. We conclude with an outlook how circRNA dysregulation 
affects disease, and how the stability of circRNAs might be exploited in 
biomedical applications.