5 MicroRNAs in Disease and Health: Diagnostic and Therapeutic Potentials Mohammad Reza Noori-Daloii 1 and Azim Nejatizadeh 2 1 Department of Medical Genetics, Faculty of Medicine, Tehran University of Medical Sciences; 2 Research Center for Molecular Medicine, Faculty of Medicine, Hormozgan University of Medical Sciences, Iran 1. Introduction In the past decade, relevance and application of genomics and proteomics technologies in early detection of diseases have verified that numerous categories of diseases could be diagnosed at early stage which would be helpful in initiation of treatment. As a result of human genome studies, a shift has occurred from mRNAs to noncoding RNAs as a main regulator of human genome. The sequence analyses of genomes in eukaryotes indicated that simple unicellular organisms, invertebrates and mammals have approximately 25%, 80% and 98%, respectively, of their genomes composed of noncoding DNA regions [1]. In higher eukaryotic organisms, approximately, the entire genome is transcribed as consisting of rRNA, tRNA, introns, 5' and 3' untranslated regions, and microRNAs. It also has been proposed that mammalian miRNAs are originated from transposons and repeats [2,3]. Moreover, it is showed that miRNAs could be developed from pseudogenes [4]. Exisisting analyses of the human genome reflects that the protein coding genes are low as 23500, indicating that a large segment of human genome consists of non-coding protein genes. Open reading Frames comprise less than 2%, repetitive sequences around 46% and non- coding parts of protein-coding genes (introns, 5' and 3'-UTRs) around 25-27% of 3.2 billion bases in the human genome [5,6]. As the human genome and its functions are being explored, the roles of non-coding RNAs are becoming more evident in specific cellular functions. Members of non-coding RNAs include microRNAs and small nucleolar RNAs which are believed to have well preserved functions in various species. MiRNAs are functional molecules that have recently emerged as important regulators of gene expression at the posttranscriptional and translational levels. Endogenous miRNAs are involved in a variety of physiological and pathological processes in human. Target mRNA specification is defined by sequence complementarity between the seed sequence and the target mRNA. The technological advances and applications of functional RNA molecules for medicine provide important insights into molecular mechanisms affecting human health and disease and could eventually lead to the discovery of diagnostic biomarkers and the development of novel gene therapies [7]. In this chapter, we first address the synthesis, mechanisms of action and functions of miRNAs. Then, we focus on recent advances and technologies in miRNA. Next, we discuss www.intechopen.com