Pablo Ross of the University of California, Davis, inserts human stem cells into a pig embryo as part of experiments to create chimeric embryos. Rob Stein/NPR hide caption

A chalkboard "bucket list" stirred imaginations and got people talking at an Indianapolis festival designed to help make conversations about death easier. Jake Harper/WFYI hide caption

Nobel laureate David Baltimore of Caltech speaks to reporters at the National Academy of Sciences international summit on human gene editing, on Tuesday in Washington, D.C. Hundreds of scientists and ethicists from around the world debating how to deal with technology that makes it easy to edit the human genetic code. Susan Walsh/AP hide caption

Ken (left) and Henry were created using DNA plucked from a skin cell of Melvin, the beloved pet of Paula and Phillip Dupont of Lafayette, La. Edmund D. Fountain for NPR hide caption

Cambrian Genomics says that what it calls a DNA printer is essentially a DNA sorter — it quickly spots and collects the desired, tailored stretch of DNA. Courtesy of Cambrian Genomics hide caption

Along with sperm, the in vitro procedure adds fresh mitochondria extracted from less mature cells in the same woman's ovaries. The hope is to revitalize older eggs with these extra "batteries." But the FDA still wants proof that the technique works and is safe. Chris Nickels for NPR hide caption

The size of the brain of a chimpanzee (right) is considerably smaller than that of a human brain. Probably multiple stretches of DNA help determine that, geneticists say. Science Photo Library/Corbis hide caption

Surgeons at Methodist University Hospital in Memphis prepare to transplant a liver in 2010. Karen Pulfer Focht/The Commercial Appeal/Landov hide caption

U.S. Marine Sgt. Robert Scoggin gets a vaccination against smallpox in 2003 at Camp Pendleton in California — one of the final steps before deployment overseas. David McNew/Getty Images hide caption

Being able to insert the two man-made letters into DNA, alongside the usual four-letter alphabet, could teach old cells new tricks and lead to better drugs, researchers say. courtesy of Synthorx hide caption