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The FDA has sent a letter requesting that John Zhang, the first person to create a three-parent baby, stop marketing his work, according to the Washington Post. Although Zhang has stated that he will not be using the technique in the U.S. without FDA permission, he continues to promote the service on his website.

How it worked: Humans have two separate sets of DNA: nuclear DNA, the 23 chromosomes we get from our mothers and fathers, and mitochondrial DNA, which is passed on in a woman's egg. Zhang used a technique called spindle transfer to place the parents' nuclear DNA into a donated egg with healthy mitochondria.

The biological background: Just like you can get genetic diseases in nuclear DNA, there are also genetic diseases in mitochondrial DNA. These mitochondrial illnesses can be severe, and sometimes fatal. The mother Zhang worked with had lost two infants and had several prior miscarriages due to the mitochondrial illness she carried.

Some context: Right now, the FDA can't use government funds to evaluate research that introduces inheritable changes into human genes. Since mitochondrial DNA can be passed on by women, clinical trials of three-parent babies aren't allowed. Despite this, in September of 2016, Zhang announced that he'd helped a woman with a mitochondrial disorder conceive an apparently healthy child. While Zhang is based in the U.S., the child was born in Mexico.

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