For the first time, researchers have permanently cured a genetic disease in an adult animal by editing the responsible gene. "In this study [doi:10.1038/nbt.2884], researchers attacked a disease called hereditary tyrosinemia, which stops liver cells from being able to process the amino acid tyrosine. It is caused by a mutation in just a single base of a single gene on the mouse (and human) genome, and prior research has confirmed that fixing that mutation cures the disease. The problem is that, until now, such a correction was only possible during early development, or even before fertilization of the egg. An adult body was thought to be simply too complex a target."