Circular RNAs (circRNAs) are currently classed as non-coding RNA (ncRNA) that, 
unlike linear RNAs, form covalently closed continuous loops and act as gene 
regulators in mammals. They were originally thought to represent errors in 
splicing and considered to be of low abundance, however, there is now an 
increased appreciation of their important function in gene regulation. circRNAs 
are differentially generated by backsplicing of exons or from lariat introns. 
Unlike linear RNA, the 3' and 5' ends normally present in an RNA molecule have 
been joined together by covalent bonds leading to circularization. 
Interestingly, they have been found to be abundant, evolutionally conserved and 
relatively stable in the cytoplasm. These features confer numerous potential 
functions to circRNAs, such as acting as miRNA sponges, or binding to 
RNA-associated proteins to form RNA-protein complexes that regulate gene 
transcription. It has been proposed that circRNA regulate gene expression at the 
transcriptional or post-transcriptional level by interacting with miRNAs and 
that circRNAs may have a role in regulating miRNA function in cancer initiation 
and progression. circRNAs appear to be more often downregulated in tumor tissue 
compared to normal tissue and this may be due to (i) errors in the back-splice 
machinery in malignant tissues, (ii) degradation of circRNAs by deregulated 
miRNAs in tumor tissue, or (iii) increasing cell proliferation leading to a 
reduction of circRNAs. circRNAs have been identified in exosomes and more 
recently, chromosomal translocations in cancer have been shown to generate 
aberrant fusion-circRNAs associated with resistance to drug treatments. In 
addition, though originally thought to be non-coding, there is now increasing 
evidence to suggest that select circRNAs can be translated into functional 
proteins. Although much remains to be elucidated about circRNA biology and 
mechanisms of gene regulation, these ncRNAs are quickly emerging as potential 
disease biomarkers and therapeutic targets in cancer.