Circular RNAs: Characteristics, function, and role in human cancer.

Circular RNAs (circRNAs) are universal and diverse endogenous noncoding RNAs (ncRNAs) that are widely found in eukaryotic cells. They form a covalently closed, continuous stable loop structure without 3' or 5' tails, which are different from linear RNAs. As new members of the ncRNAs, circRNAs' formation, function, and mechanism are attracting increased research attention. CircRNAs play important roles in all kinds of cancer and may be potential novel biomarkers and therapeutic targets for cancer treatment through their function as microRNA (miRNA) molecular "sponges", RNA-binding protein (RBP) sponges, protein translators, and gene transcription regulators. In this review, we introduce the formation and function of circRNAs, and summarize the biological effects of circRNA in tumorigenesis and progression, providing evidence for the potential use of circRNAs in the diagnosis and clinical treatment of cancer.