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

itoggle caption Karen Pulfer Focht/The Commercial Appeal/Landov

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

itoggle caption David McNew/Getty Images

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

itoggle caption courtesy of Synthorx

Botulism bacteria, or Clostridium botulinum, grow in poorly preserved canned foods, especially meat and fish. The microbe's toxin could be lethal as a bioweapon. Dr. Phil Luton/Science Photo Library/Corbis hide caption

itoggle caption Dr. Phil Luton/Science Photo Library/Corbis

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

The research team used yeast chromosome No. 3 as the model for their biochemical stitchery. Pins and white diamonds in the illustration represent "designer changes" not found in the usual No. 3; yellow stretches represent deletions. Lucy Reading-Ikkanda hide caption

itoggle caption Lucy Reading-Ikkanda

Erick Munoz stands by a photo of his wife, Marlise Munoz, at home in Fort Worth, Texas, on Jan. 3. She is being kept on life support in a local hospital against the family's wishes. Fort Worth Star-Telegram/MCT via Getty Images hide caption

itoggle caption Fort Worth Star-Telegram/MCT via Getty Images

Directors and bioethicists hashed out how moral medical issues should be depicted on screen during a meeting in Los Angeles. Courtesy of Colin Crowley hide caption

itoggle caption Courtesy of Colin Crowley

The botulism toxin comes from Clostridium botulinum bacteria, seen here in a colorized micrograph. James Cavallini/Science Source hide caption

itoggle caption James Cavallini/Science Source

A warning to us all Universal/The Kobal Collection hide caption

itoggle caption Universal/The Kobal Collection

What should parents be told before their premature infants participate in a clinical study? iStockphoto.com hide caption

itoggle caption iStockphoto.com

In 2008, Michael Mastromarino was sentenced in a New York City courtroom for enterprise corruption, body stealing and reckless endangerment. Jesse Ward/AP hide caption

itoggle caption Jesse Ward/AP

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

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