Circular RNAs (circRNAs) are a novel class of regulatory RNAs that despite being 
relatively abundant have only recently begun to be explored. There are many 
thousands of genes that appear capable of producing circRNAs, however the 
function of all but a handful remain to be determined. What is emerging about 
these highly conserved molecules is that they play important roles in biology 
and cancer biology in particular. The most explored function of circRNAs is as 
master regulators of gene expression that act to sequester or ´sponge´ other 
gene expression regulators, in particular miRNAs. They have also been 
demonstrated to function via direct modulation of transcription, and by 
interfering with splicing mechanisms. Although generally expressed in low 
abundance when compared to their linear counterparts, they are often expressed 
in a tissue- and developmental stage- specific manner. Coupled with their 
remarkable resistance to RNAse activity due to a covalent closed cyclic 
structure, circRNAs show great promise as novel biomarkers of cancer and other 
diseases. In this review we consider the current state of knowledge regarding 
these molecules, their synthesis, function, and association with cancer. We will 
also review some of the challenges that remain to be resolved if this emerging 
class of RNAs are really to become useful in the clinic.