"By defining cellular intermediates of the reprogramming process, our work provides reference points for understanding the molecular mechanisms underlying pluripotent (iPS) cell generation," said Dr. Matthias Stadtfeld, one of researchers on the project. "For example, one can now take the whole reprogramming process apart into different stages and ask what enzymes or genes are important for the different stages to be initiated and completed." The team at Harvard Stem Cell Institute and Massachusetts General Hospital have been studying the conversion of mouse adult cells to embryonic-like states by using four genes. The general way to induce iPS cells is to use cancer causing genes to initiate the process and use retroviruses to insert the genes into the target cells - retroviruses which could activate the cancer cells. The use of either oncogenes or retroviruses would make it impossible to use these cells in patients. It is hoped the results of this study, published in Cell Stem Cell, will help create stable cells by investigating a process that had previously been a virtual black box. With a better understanding, the researchers predict more efficient chemical compounds that accelerate the process could be found or even replacements for viral vectors. "A better knowledge of the reprogramming kinetics should help to generate iPS cells without using viral vectors that stably integrate into the genome. This would significantly lower the risk of tumour formation from iPS derived cells and open the door wide for clinical application of direct reprogramming," said Stadtfeld. The time markers of the process could also provide a crucial insight into developing usable cells, as certain chemicals become dangerous after a known length of time. "The importance of this finding is that it will tell us how long we need to throw chemicals or proteins on the cells for the programming to be effective. It could have been that these viruses are only necessary for two days, or three weeks," said Konrad Hochedlinger, another author of the paper. "If you know a certain chemical, or protein, becomes dangerous after 10 days, but you'll only need to use it for eight days, you have learned something important. Up to this point it was unknown what the sequence of events occurring was. But using these markers, we have been able to define what happens during reprogramming." The team hopes the work will be a platform for a detailed dissection of how pluripotent cells (such as iPS cells) are generated from different adult cells (such as fibroblasts). Stadtfeld adds, "This reprogramming framework outlined by us can now be further refined by both biochemical and genetic approaches."