Dear Editor,

Mouse and human somatic cells can be induced to become pluripotent stem (iPS) cells by retroviral transduction with defined transcription factors 1, 2, 3. Reprogrammed pluripotent stem cells have great potential for regenerative medicine and also provide a good experimental system for studying epigenetic reprogramming and differentiation. However, at present, the reprogramming process is still somewhat slow and ineffective. Research to improve reprogramming efficiency has included co-expression with more factors or treatment with chemical compounds 4, 5, 6. Here we report that the pace and efficiency of reprogramming can be greatly improved by using a high concentration of knockout serum replacement (KOSR) instead of fetal bovine serum (FBS) in the culture medium. While reprogramming efficiency is increased more than 100-fold, the iPS cell lines generated with this modified culture condition maintain normal karyotype, hold full developmental potential including germ-line contribution in diploid chimeric mice, and support full-term development of tetraploid complementation embryos.

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