MicroRNA (miRNA) are a class of short non-coding RNAs that regulate gene expression at the posttranscriptional level. They are involved in key cellular processes and development as well as progression of many diseases. Their levels reflect the physiological state of organisms; therefore, the expression profiles of these molecules are analyzed in bio-marker studies. Due to their properties, miRNA appear to be promising dia-gnostic, prognostic and predictive bio-markers of cancer. Recent studies

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... ndicate the existence of sequence variants in miRNA, so-called isomiRs, which differ from the annotated miRNAs by altered sequences due to posttranscriptional modifications. These isomiRs may have a higher abundance than canonical miRNA. The characterization of isomiRs reveals their regulated distribution and different bio-logical properties and thus suggest the possible bio-logical significance of the modifications. The presence of isomiRs can also significantly affect the results of bio-marker studies. Currently, the research is focused on their possible clinical significance. The aim of this review is to provide an overview of current knowledge about sequence variants in miRNA. The review summarizes the mechanisms of isomiRs bio-genesis and describes the effects of sequence heterogeneity on miRNA stability, function and analysis. Subsequently, the role of isomiRs in bio-marker studies is discussed.