Circular RNAs (circRNAs) are a new class of non-coding RNAs formed by covalently 
closed loops through backsplicing. Recent methodologies have enabled in-depth 
characterization of circRNAs for identification and potential functions. 
CircRNAs play important roles in various biological functions as microRNA 
sponges, transcriptional regulators and combining with RNA binding proteins. 
Recent studies indicated that some cytoplasmic circRNAs can be effectively 
translated into detectable peptides, which enlightened us on the importance of 
circRNAs in cellular physiology function. Internal Ribosome Entry site (IRES)- 
and N6-methyladenosines (m6A)-mediated cap-independent translation initiation 
have been suggested to be potential mechanism for circRNA translation. To date, 
several translated circRNAs have been uncovered to play pivotal roles in human 
cancers. In this review, we introduced the properties and functions of circRNAs, 
and characterized the possible mechanism of translation initiation and 
complexity of the translation ability of circRNAs. We summarized the emerging 
functions of circRNA-encoded proteins in human cancer. The works on circRNA 
translation will open a hidden human proteome, and enhance us to understand the 
importance of circRNAs in human cancer, which has been poorly explored so far.