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

Ryoji Noyori, a Nobel Prize-winning chemist and president of Japan's prestigious RIKEN research institute, bows at a news conference in Tokyo Tuesday to apologize for the scientific misconduct of a RIKEN colleague. Eugene Hoshiko/AP hide caption

itoggle caption Eugene Hoshiko/AP

A mouse embryo grows from stem cells made by stressing blood cells with acid. The blood cells are tagged with a protein that creates green light. Courtesy of Haruko Obokata hide caption

itoggle caption Courtesy of Haruko Obokata

After President Obama overturned Bush-era policy restricting federal funding of embryonic stem cell research in 2009, Nebraska Right to Life led a protest of the research outside the University of Nebraska regents' meeting. Nati Harnik/AP hide caption

itoggle caption Nati Harnik/AP

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

A scientist removes the nucleus from a human egg using a pipette. This is the first step to making personalized embryonic stem cells. Courtesy of OHSU Photos hide caption

itoggle caption Courtesy of OHSU Photos