A Greek woman gave birth to a baby with three parents in a successful clinical trial to test the controversial procedure on Wednesday in Spain. The treatment, maternal spindle transfer (MST), puts DNA from the mother’s egg into a donor’s egg prior to its fertilization by sperm. The new mother previously had four failed IVF attempts due to a mitochondrial issue that had inhibited her from having a healthy pregnancy. The ethics of the procedure, which is banned in the U.S., has been questioned, but researchers think this could be a breakthrough fertility treatment in preserving a mother’s genetic material. A “three-parent baby” was previously born to a couple suffering from genetic disease in Mexico in 2016.