From the article: "There are two procedures to avoid passing on faulty mitochondria. The first involves using an egg from one woman with mitochondrial defects and the sperm of the father. Scientists then put that embryo into an emptied egg from a second woman with healthy mitochondria. The DNA from the second woman amounts to less than 1 percent of the embryo's genes.
In the second technique, scientists transfer nuclear DNA out of a day-old embryo with defective mitochondria. The DNA is implanted into another single-cell embryo with normal mitochondria. The nuclear DNA from the donor embryo is discarded, leaving the healthy mitochondria."