microRNAs (miRNAs) are small, nonprotein-coding RNAs that function as posttranscriptional regulators of target genes. miRNAs are involved in multiple cell differentiation, proliferation, and apoptosis processes that are closely related to tumorigenesis. Circulating miRNAs are promising cancer biomarkers under development with great translational potential in personalized medicine. Here, we describe the origin and function of circulating miRNAs and compare the current new high-throughput technology applied to miRNA quantitation. The latest publications on circulating miRNAs were summarized, indicating that miRNAs are potential biomarkers of diagnosis, prognosis, and treatment response of major cancer types including prostate, breast, lung, colorectal, and hematological cancers. We addressed the strengths and limitations of applying circulating miRNAs in clinical laboratory and several issues associated with the accurate measurement of circulating miRNAs.
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