Evidence has emerged recently indicating that differentiation is not entirely a one-way process, and that it is possi ble to convert one cell type to ano ther, both in vitro and in vivo. This phenomenon is called trans different iation, and is generally defin ed as the stable switch of one cell type to an other. Tra nsdifferen tiatio n plays crit ical roles d uring development and in regeneration pathways in nature. Althoug h this phenom enon occurs rarely in na ture, recent studies have bee n focuse d on trans different iation and the reprogramming ability of cells to produce specific cells with ne w p he no typ es f or us e in ce ll t he rap y an d re gen er ativ e medic ine. Thus, unders tanding the p rinciples and the mech-anism of this proces s is importan t for produci ng desired cell types . Here some well-docume nted examp les of trans differ-entiation, and their significance in development and regener-ation are reviewed. In addition, transdifferentiation pathways are con sidered and their potential mol ecular mecha nisms, especially the role of master switch genes, are considered. Finally, the significance of transdifferentiation in regenerative medicine is discussed.

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