Mitochondrial replacement therapy (MRT) is an experimental assisted reproductive technology (ART) for women with mitochondrial disease who want to avoid passing that disease on to their children and for women with certain causes of infertility. It is unique in that it results in a child with DNA from three different people. Our cells, including egg and sperm cells, contain both nuclei with nuclear DNA—the “instruction manual” for the cell—and many intracellular organelles that carry on the functions of our cells—the “machinery” of the cell. One of those organelles, mitochondria, has its own small, independent source of DNA called mitochondrial DNA. MRT essentially works by transferring the nucleus (containing nuclear DNA) from one egg cell (the “original egg”) into a different egg cell (the “donor egg”) that has had its nucleus removed but retains its mitochondria and associated mitochondrial DNA. Because the fertilized embryo contains mitochondrial DNA from the donor egg that is different from the mitochondrial DNA in the original egg, the resulting embryo has DNA from three different sources: nuclear DNA from the original egg, mitochondrial DNA from the donor egg, and nuclear DNA from the sperm. . . .