Three-parent IVF, which involves transferring the nucleus of an egg from one woman into the cytoplasm of a younger woman's egg with healthy mitochondria, is legal in the UK and other countries but not the US.

“What I'm about to describe is not legal in the US, it is illegal, but is legal in the UK and in other countries, is this notion of three parent IVF, where it does seem that some of the eggs that persist in older females, even if fertilized, don't produce healthy embryos.”