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However, instead of using Fbx15 to select for pluripotent cells, the researchers used Nanog, a gene that is functionally important in ESCs.
By using this different strategy, the researchers created i PSCs that were functionally identical to ESCs.
While this combination is most conventional in producing i PSCs, each of the factors can be functionally replaced by related transcription factors, mi RNAs, small molecules, or even non-related genes such as lineage specifiers.
i PSC derivation is typically a slow and inefficient process, taking 1–2 weeks for mouse cells and 3–4 weeks for human cells, with efficiencies around 0.01%–0.1%.
Reprogramming of human cells to i PSCs was reported in November 2007 by two independent research groups: Shinya Yamanaka of Kyoto University, Japan, who pioneered the original i PSC method, and James Thomson of University of Wisconsin-Madison who was the first to derive human embryonic stem cells.
With the same principle used in mouse reprogramming, Yamanaka's group successfully transformed human fibroblasts into i PSCs with the same four pivotal genes, OCT4, SOX2, KLF4, and C-MYC, using a retroviral system, The generation of i PS cells is crucially dependent on the transcription factors used for the induction.
Oct-3/4 and certain products of the Sox gene family (Sox1, Sox2, Sox3, and Sox15) have been identified as crucial transcriptional regulators involved in the induction process whose absence makes induction impossible.
While the i PSC technology has not yet advanced to a stage where therapeutic transplants have been deemed safe, i PSCs are readily being used in personalized drug discovery efforts and understanding the patient-specific basis of disease.
The table at right summarizes the key strategies and techniques used to develop i PS cells in the first five years after Yamanaka et al.'s 2006 breakthrough.
Rows of similar colors represent studies that used similar strategies for reprogramming.
However, considerable advances have been made in improving the efficiency and the time it takes to obtain i PSCs.
Upon introduction of reprogramming factors, cells begin to form colonies that resemble pluripotent stem cells, which can be isolated based on their morphology, conditions that select for their growth, or through expression of surface markers or reporter genes.