Circular RNAs (circRNAs) are long, non-coding RNAs that result from the 
non-canonical splicing of linear pre-mRNAs. However, the characteristics and the 
critical role of circRNA in co-/post-transcriptional regulation were not well 
recognized until the "microRNA sponge" function of circRNA is discovered. Recent 
studies have mainly been devoted to the function of the circular RNA sponge for 
miR-7 (ciRS-7) and sex-determining region Y (SRY) by targeting microRNA-7 
(miR-7) and microRNA-138 (miR-138), respectively. In this review, we illustrate 
the specific role of circRNAs in a wide variety of cancers and in regulating the 
biological behavior of cancers via miR-7 or miR-138 regulation. Furthermore, 
circRNA, together with its gene silencing ability, also shows its potential in 
RNA interference (RNAi) therapy by binding to target RNAs, which provides a 
novel perspective in cancer treatment. Thus, this review concerns the 
biogenesis, biological function, oncogenesis, progression and possible therapies 
for cancer involving circRNAs.