Researchers have shown that cloned embryos undergo complete nuclear reprogramming and resemble fertilized embryos. The report, appearing in the December 6 issue of Proceedings of the National Academy of Science, appears to refute the long-held belief that the high failure rate of cloning is due to faulty nuclear reprogramming of the donor cell nucleus to an embryonic state. Researchers suggest that the findings may only deepen the mystery of why cloning often fails, and demonstrate how far scientists have to go before improving the process.