Although tissue miRNA expression could serve as molecular classifier for clinical outcome, there are several drawbacks including small quantities of diagnostic tumor tissue, genetic tumor heterogeneity in space and time, and unavailable acquisition of repeated biopsies. Therefore, circulating biomarkers offer a non-invasive opportunity for early diagnosis, prognosis evaluation and drug response prediction in cancer. Circulating miRNAs are stable and reproducible in blood due to their incorporation in exosomes/microvesicles or protein complexes, protecting them from degradation by endogenous RNase [109, 110]. Technical advances in detection and profiling makes them promising candidates. Recently, the occurrence of miRNAs in the serum or plasma of human has been repeatedly observed and become the focus of biomarker discovery.