U.S. and British government agencies are moving toward enabling what would be the first clinical trials of what is, in effect, human germline engineering – genetic modifications that would be passed down through the generations. The modifications would not be made to nuclear DNA. Instead, the procedure being considered is oocyte modification – creating an egg cell with the nuclear DNA of one woman and the mitochondrial DNA of another. The goal would be to allow women with mitochondrial diseases to have babies that share their nuclear DNA, but are not at risk of inheriting their mitochondrial disease. The UK...