2 function of miRNAs is extensively studied in the near future to identify their potential clinical applications. 2. MicroRNA Biogenesis miRNA biogenesis has been studied by many scientists. A schematic overview of miRNA biogenesis is shown in Figure 1. Most of the miRNA genes exist in clusters in the genome and are polycistronically expressed from their own promoter. Other miRNA genes are found in intronic regions and are transcribed as a part of annotated genes. The transcription of most miRNA genes is mediated by RNA polymerase II (Pol II), producing long primary miRNAs (pri- miRNAs) that are capped at the 5 end and polyadenylated at the 3 end. These pri-miRNAs contain a stem of appro- ximately 33 base pairs, a terminal loop, and ﬂanking ssRNA segments [14]. However, a small group of miRNAs associated with Alu repeats can be transcribed by Pol III [15]. Two steps of ribonuclease pro-cessing reactions are required to generate mature miRNAs. The ﬁrst step occurs in the nucleus and involves the release of a 70 nt intermediate hairpin structure (pre-miRNA) from the RNA duplex in the pri-miRNA by the RNase III-type protein, Drosha [16]. Moreover, Drosha-mediated pri-miRNA processing requires the cofactor, DiGeorge syndrome critical region gene 8 (DGCR8). Together with DGCR8, Drosha forms a large complex known as the microprocessor complex, which is approximately 650 kDa in humans [17, 18]. Speciﬁcally, DGCR8 interacts with pri-miRNAs through the ssRNA segments and the stem (approximately 33 bp), and DGCR8 assists Drosha to cleave the substrate (approximately 11 bp) from the ssRNA/dsRNA junction [19, 20]. The resulting pre-miRNA is transported out of the nucleus and into the cytoplasm by Exportin-5 and its cofactor, Ran-GTP [21]. Apart from the canonical miRNA pathway, an alternative nuclear pathway for miRNA biogenesis has been recently described in invertebrates [22, 23] and mammals [24]. This noncanonical pathway involves short introns with hairpin potential, which are termed mirtrons. Mirtrons bypass are processed by the microprocessor. Therefore, they are pro- cessed by splicing and debranching. Debranched mirtrons access the canonical miRNA pathway during nuclear export. The debranched mirtrons are cleaved by Dicer and incorpo- rated into silencing complexes [22, 23]. The miRNA maturation process in the cytoplasm is carried out by Dicer, which is a highly conserved RNase III-type endoribonuclease present in almost all eukaryotic organisms. PremiRNAs are cleaved near the terminal loop by Dicer-releasing miRNA duplexes (approximately 22 nt) [25]. Human Dicer interacts with proteins, such as TAR RNA binding protein (TRBP; also known as TARBP2) [26, 27]. However, current studies have, suggested that they are not required for miRNA processing but that they contribute to the formation of the RNA-induced silencing complex (RISC) [28]. Following Dicer cleavage, the 22-nt RNA duplex binds to Argonaute (Ago) proteins to generate the eﬀector complex, RISC. One strand of the 22-nt RNA duplex remains in the Ago complex as a mature miRNA (the guide strand or miRNA), and the other strand (the passenger strand Journal of Aging Research or miRNA∗) is degraded [25]. Next, the miRNA guides RISC to speciﬁcally recognize and repress target mRNAs. In most cases, miRNAs repress protein expression through base pairing with the 3 -UTRs of the target mRNA [28]. Perfect complementarity, which is rare in animal miRNA/mRNA base pairing, allows Ago-catalyzed cleavage of the mRNA strand. In contrast, central mismatches exclude cleavage and promote repression of mRNA translation. The speciﬁcity of miRNA targeting is deﬁned by Watson- Crick complementarities between positions 2 to 8 from the 5 -miRNA (also known as the seed) with the 3 -UTR of the target mRNAs. When miRNAs and their target mRNA sequence have perfect complementarities, RISC induces mRNA degradation. When an imperfect miRNA/mRNA tar- get pairing occurs, protein translation is blocked. Regardless which of these two events occurs, the net result is a decrease in the amount of proteins encoded by the mRNA targets. 3. miRNAs Involved in Senescence Cellular senescence was originally described in primary cells as a process that limits the replicative potential of human diploid ﬁbroblasts in culture. This type of senescence is called replicative senescence. Senescence is an important block to cell cycle progression during the aging of cells in culture and is a fundamental barrier that cells must bypass during carcinogenesis. Senescent cells are characterized by the expression of β-galactosidase, overexpression of plasminogen activator protein 1 (PAI-1), and altered cell morphology characterized by a giant cell size, increased cytoplasmic granularity, and a single large nucleus [29]. Cellular senescence is a process that is triggered by several types of stresses as follows: telomeric erosion resulting from repeated cell division (replicative senescence); DNA damage; oxidative stress resulting from mitochondrial deterioration; overexpression of oncogenes; loss of tumor suppressors such as PTEN and VHL (oncogene-induced senescence; OIS) [29, 30]. OIS was ﬁrst observed when an oncogenic form of Ras (e.g., RasG12V), which is a cytoplasmic transducer of mitogenic signals, was expressed in normal human ﬁbrob- lasts [31]. Other members of the Ras signaling pathway, such as v-raf-1 murine leukemia viral oncogene homolog 1 (RAF), mitogen-activated protein kinase kinase 1 (MEK), v-mos Moloney murine sarcoma viral oncogene homolog (MOS), and v-raf murine sarcoma viral oncogene homolog B1 (BRAF), in addition to pro-proliferative nuclear proteins, such as the E2F transcription factor, can also induce a senescence response upon overexpression [32]. Moreover, OIS can be caused by the loss of the tumor suppressors that function upstream of oncogenes, such as phosphatase and tensin homolog (PTEN), von Hippel-Lindau tumor sup- pressor (VHL), and neuroﬁbromin 1 (NF1), resulting in an increase of oncogenic signaling that leads to senescence [30]. In general terms, OIS has similar characteristics to replicative senescence including the presence of β-galactosidase-positive cells, induction of cell cycle inhibitory proteins, and pheno- typic morphology of giant cells. The major pathways that regulate cellular senescence are the p53/p21Cip1 and p16INK4A-pRB tumor suppressor