Choice of samples in extracellular microRNA research: Which fraction is better- exosomal or nonexosomal?

MicroRNAs (miRNAs) are a species of noncoding RNA that posttranscriptionally regulates the expression of target mRNAs via degradation or translational repression. Recently, miRNAs in biofluids got extraordinary research priority in the field of noninvasive biomarker development because of their association with pathophysiological events, noninvasive way to obtain, and a remarkable stability in the extracellular harsh environment. Although several hypotheses have been proposed, the precise mechanisms of cellular release of extracellular miRNAs are not properly understood. In addition, it is often confusing to the researchers, which fraction of biofluid (based on release mechanisms) is suitable for biomarker development research. This review will briefly introduce extracellular miRNAs (ECmiRNAs) and their release mechanisms. Furthermore, based on recent evidence, it will also summarize and suggest the appropriate fraction of biofluid to work with for biomarker development.