Classified review of circular RNA databases and web-based resources

Circular RNAs (circRNAs) are a class of non-coding RNAs, with a covalently closed loop structure, and unlike linear RNAs, they don't have free 5' cap or 3' polyA tails. A considerable proportion of circRNAs are conserved among different species and some of them are tissue-specific or developmental stage-specific. They have various pivotal roles in biological systems such as miRNA sponging, protein binding and etc. In the last two decades, different experimental and computational methods have been developed to discover circRNAs and reveal their function. This leads to a pile of information about circRNAs and their functions. Databases play an important role in storing and classification of these data and using these databases at each level of a study could be a time-saver for researchers. There are several available databases and each one of them covers specific aspects of circRNAs and it's important to choose a suitable database for a specific task. Here, we have classified different aspects of circRNAs that are covered by these databases, and reviewed current circRNA databases. We also compared these databases according to the types of data they are providing. This review will be a useful reference for researchers, especially those with a biology background, to choose the right tools and follow the provided instructions for their studies. Additionally, it will help in identifying gaps in the current resources and inspire future developments in this area.