Figure 20. Mitochondrial replacement therapy (‘three-parent baby’).

Eggs are harvested (1) from the mother-to-be, whose mitochondria are affected by a pathogenic DNA variant. The nucleus is extracted from the egg (2). Eggs are also harvested (3) from a donor female who has healthy mitochondria, and the nucleus is removed from the egg, leaving only the cytoplasm (4). Now the nucleus (2) is injected into the enucleated egg (4) to generate an egg (5) that has the nuclear DNA of the mother-to-be, and healthy mitochondria. This can now be fertilised in vitro using sperm from the father (6), and allowed to develop for a few days before implantation into the mother-to-be.