This mouse egg (top) is being injected with genetic material from an adult cell to ultimately create an embryo — and, eventually, embryonic stem cells. The process has been difficult to do with human cells. James King-Holmes/Science Source hide caption

itoggle caption James King-Holmes/Science Source

A file photo from 2011 shows a man touching a giant bronze sculpture of a mammoth in the Siberian city of Khanty-Mansiysk. A team of Russian and South Korean scientists who found a well-preserved frozen woolly mammoth carcass this month say it also included blood. Natalia Kolesnikova/AFP/Getty Images hide caption

itoggle caption Natalia Kolesnikova/AFP/Getty Images

Human embryos grow in a petri dish two days after scientists in Oregon cloned them from a donor's skin cell. http://www.flickr.com/photos/ohsunews/8726915230/in/photostream//Courtesy of OHSU Photos hide caption

itoggle caption http://www.flickr.com/photos/ohsunews/8726915230/in/photostream//Courtesy of OHSU Photos