**5. Clinical implication of miRNAs**

MicroRNAs are becoming well recognized as their expression levels are changed in different diseases such as cancer, liver disease, coronary heart ischemic disease, and metabolic disease. Some miRNAs are increased, whereas others are decreased in a specific disease, creating a signature miRNA pattern that could serve as a biomarker or molecular therapeutic target for a particular disease. For example, in the case of cancers, the overexpressed miRNAs are commonly considered as oncogenes. On the other hand, some of the miRNAs are expressed very low levels in certain cancerous cells and usually prevent tumor development. These groups of miRNAs are called as tumor suppressor miRNAs. Let-7 is one of these tumor suppressor miRNAs [27]. The miRNAs have remarkable potential in the clinical arena because they can be detected in the blood, serum, tissues (fresh and formalin-fixed paraffin-embedded), and fine-needle aspirate specimens. Recently, novel in situ hybridization techniques have been described to detect miRNAs in tissues, which enables direct miRNA and histomorphologic correlation [28]. The clinical implications of miRNA use in medicine are present in both diagnostic and therapeutic approaches. In miRNA-based diagnostics, miRNA expression profiling has been evaluated as a reliable diagnostic biomarker for differentiating between normal and tumor specimens [29, 30]. Ali et al. have reported that the expression of let-7c, let-7f, and miR-200c were significantly decreased in pancreas cancer patients whereas miR-486-5p and miR-451 were significantly increased in those patients compared with the normal people, which suggests that these miRNAs can be served as signature biomarkers for pancreas cancer [30]. Additionally, circulating miRNAs can be employed in diagnostic strategies to detect for alterations associated with either disorder affliction or treatment response. In the miRNA-based therapeutics, the miRNA expression is altered in different diseases and it is now feasible to correct miRNA expression by injecting miRNAs similar to the use of antisense mRNAs and RNAi. For example, because the activation of onco-miRNA genes could cause development of cancer, artificial antisense miRNAs could be synthesized and used to inhibit their targeted miRNAs to treat or prevent the cancer.
