ABSTRACT Regenerative medicine using pluripotent/multipotent stem cells holds a great promise in developing therapies for treating developmental abnormalities, degenerative disor- ders, and aging-related illness. However, supply and safety of the stem cells are two major problems with today’s re- generative medicine. Recent development of induced pluri- potent stem cells (iPSCs) has overcome the supply shortages by allowing the reprogramming of patients’ body cells to embryonic stem cell (ESC)-like pluripotent cells. Still, the potential tumorigenicity of iPSCs remains as an obstacle. During early embryogenesis ESCs can be generated without tumor formation; therefore, under- standing the mechanisms underlying ESC generation may help us to prevent iPSC tumorigenicity. Previous studies have shown that an ESC-enriched noncoding RNA, miR- 302, induces somatic cell reprogramming (SCR) to form iPSCs, suggesting its pivotal role in stem cell generation. Recent research further revealed that miR-302-induced In the past, it was widely assumed that a stem cell, once dif- ferentiated, could not revert back to an earlier developmental stage. The recent discovery of iPSCs, however, revokes this concept and provides the ﬁrst evidence that there is an inter- nal mechanism capable of reprogramming the ‘‘stemness’’ of a differentiated tissue cell back to an ESC-like pluripotent state, indicating a fountain of youth intrinsic to every cell in the body. A stem cell has two principal abilities of stemness: (1) self-renewal in which it can multiply through unlimited division and (2) pluripotency in which it can differentiate into a variety of tissue cells originating from all three embryonic germ layers, the ectoderm, mesoderm, and endoderm. The breakthrough discovery of such a reprogramming event pro- vides us a powerful means to generate and regenerate unlim- ited pluripotent stem cells directly from the resourceful pool of body tissue cells. Yet, the involved mechanism, called so- matic cell reprogramming (SCR), remains elusive. SCR was ﬁrst observed by transferring somatic cell nuclei into the cytoplasm of oocytes, which forms ESC-like hybrid cells that can develop into animal clones possessing the same genetic traits as the hosts of the somatic cell nuclei [1, 2]. Disclosure of potential conﬂicts of interest is found at the end of this article. EMBRYONIC STEM CELLS/INDUCED PLURIPOTENT STEM CELLS Concise Review: Deciphering the Mechanism Behind Induced Pluripotent Stem Cell Generation SHI-LUNG LIN Division of Regenerative Medicine, WJWU & LYNN Institute for Stem Cell Research, Santa Fe Springs, California, USA Key Words. Induced pluripotent stem cell • Mechanism • Somatic cell reprogramming • DNA demethylation • Pluripotency • miR-302 • microRNA Author contributions: S.-L.L.: conception and design, ﬁnancial support, provision of study materials, collection and assembly of data, data analysis and interpretation, manuscript writing, and ﬁnal approval of manuscript. Correspondence: Shi-Lung Lin, Ph.D., WJWU & LYNN Institute for Stem Cell Research, Santa Fe Springs, California, USA. Telephone: 626-236-2885; Fax: 626-294-9950; e-mail: shilungl@mirps.org Received June 15, 2011; accepted for publication September 10, 2011; ﬁrst published online in STEM CELLS EXPRESS September 21, 2011. VC AlphaMed Press 1066-5099/2009/$30.00/0 doi: 10.1002/stem.744 STEM CELLS 2011;29:1645–1649 www.StemCells.com SCR involves an epigenetic reprogramming mechanism similar to the natural zygotic reprogramming process in the two- to eight-cell-stage embryos. These ﬁndings indi- cate that miR-302, as a cytoplasmic gene silencer, inhibits the translation of multiple key epigenetic regulators, including AOF1/2, methyl-CpG binding proteins 1 and 2, and DNA (cytosine-5-)-methyltransferase 1, to induce global DNA demethylation, which subsequently triggers the activation of the previously deﬁned factors Oct4, Sox2, and Nanog to complete the reprogramming process. The same mechanism was also found in the event of somatic cell nuclear transfer. Based on these advanced understand- ings, this review describes the currently established SCR mechanism—as compared to the natural process of early ESC formation—and demonstrates how stem cell researchers may use this mechanism to improve iPSC gen- eration. STEM CELLS 2011;29:1645–1649 Although this kind of somatic cell nuclear transfer (SCNT) technology has been intensively practiced for over 14 years to produce various species of animal clones, the necessity of oocytes is ethically controversial and the mechanism is unclear. In 2006, Takahashi and Yamanaka established a novel reprogramming method that bypassed any use of oocyte or embryonic components. By introducing four deﬁned tran- scription factors, Oct4, Sox2, Klf4, and c-Myc, somatic cells were reprogrammed to iPSCs that showed ESC-like properties in almost all aspects [3]. Subsequently, Yu et al. [4] also suc- cessfully generated iPSCs using another set of four deﬁned factors, Oct4, Sox2, Nanog, and Lin28. Nevertheless, with all these efforts, the SCR mechanism is still unsolved. It was not until 2 years after the discovery of iPSCs when another method of iPSC generation was found which revealed the mechanism of SCR. Lin et al. and their peers showed that a small noncoding RNA, called miR-302, can replace all pre- viously deﬁned factors to reprogram human and mouse somatic cells to ESC-like iPSCs [5–8]. It was understood that none of these human iPSCs induced by miR-302 have been tested for germline transmission in either chimera or clones