Can't remember if I already shared the above picture... it's my favorite sunset shot so far, so forgive me if it's a deja vu.
The Nobel Prize in medicine this year was awarded to John Gurdon and Shinya Yamanaka for pioneering the reprogramming of cells into an embryonic-like state. Embryonic stem cells are cells that undergo asymmetric division, as they divide into an undifferentiated cell and into a specialized cell. This way, they can grow indefinitely while maintaining their undifferentiated state and, at the same time, keep the ability to differentiate into all three germ layers, the cells formed during embryogenesis.
When still a PhD student, in 1958, Gurdon cloned a frog using the nucleus of a cell taken from the intestine of a tadpole. It took another 38 years before the first mammal was cloned: the first cloned sheep, Dolly, was born in 1996 from an unfertilized egg whose nucleus had been replaced with the nucleus of an adult cell. In this case, the adult cell, by being placed into the egg, was effectively "reprogrammed" into an embryonic stem cell. Up until Gurdon's work was published in 1962, general belief was that once cells specialized, they could not revert. The discovery that cells can actually undergo "reprogramming" under special circumstances is quite significant because it gives hope that we can achieve tissue regeneration and treat degenerative diseases or spinal cord injuries.
In 2006 Yamanaka and his colleague Kazutoshi Takahashi published a paper in Cell  in which they showed that, activating four genes, they were able to reprogram adult fibroblasts from mouse embryonic cells. They called the new cells induced pluripotent cells, or iPS, and found that they expressed embryonic-state cell markers. In fact, once in the proper environment, they contributed to embryonic development.
Yamanaka is a strong believer that this research will eventually lead to successful regeneration therapies. In fact, he plans to start a bank of induced pluripotent stem cells obtained from 75 different cell lines. Is this the beginning of a new era? A word of caution comes from a paper published in PNAS at the end of 2010 : in this paper, Serwold and colleagues derived mice from reprogrammed T-cells (cells from the immune system) and showed that roughly half of the mice generated this way spontaneously developed T-cell lymphomas.
The mice were generated by transferring T-cell nuclei into enucleate oocytes. As they mature, T-cells undergo genomic rearrangements, and while normally these rearrangements occur in T-cells only during a specific stage of their development, such rearrangements were observed in all somatic cells in the cloned mice. In  Serwold et al. show that these rearrangements undergo T-cell lymphomagenesis: in other words, they cause cancer. Though T-cells are not the only cells that currently can be reprogrammed, this study clearly shows that different cell lines can yield different outcomes, some quite deleterious.
"This study suggests that precautions should be taken to ensure that the identity of the reprogrammed cell of origin is known, and that T cells, and probably also B cells, are not inadvertently turned into therapeutic iPS cells. Recent studies have used human blood-derived T cells as sources of iPS cells, and these cells promise to be valuable tools for studying human immune development and disease; however, the results presented here indicate that extra caution is warranted regarding the therapeutic use of such T cell-derived iPS cells ."
 Takahashi, K., & Yamanaka, S. (2006). Induction of Pluripotent Stem Cells from Mouse Embryonic and Adult Fibroblast Cultures by Defined Factors Cell, 126 (4), 663-676 DOI: 10.1016/j.cell.2006.07.024
 Serwold, T., Hochedlinger, K., Swindle, J., Hedgpeth, J., Jaenisch, R., & Weissman, I. (2010). T-cell receptor-driven lymphomagenesis in mice derived from a reprogrammed T cell Proceedings of the National Academy of Sciences, 107 (44), 18939-18943 DOI: 10.1073/pnas.1013230107