6 years ago, the first gene-edited human babies were born. Scientist He Jiankui edited the DNA of twin girls in the embryonic stage, using CRISPR-Cas9 to alter the gene CCR5 in an attempt to render the babies resistant to contracting HIV. This was the first experiment of its kind, and resulted in international outcry and the arrest of He. But why was it so controversial?

First, a brief overview of what gene editing is. The main tool used is CRISPR-Cas9, which is basically a molecular pair of scissors and thread: first it cuts the DNA (scissors), and then inserts a new piece of DNA (thread). Once the DNA joins together with the new piece, the gene is edited. When thinking of gene therapy treatments, somatic cell editing is what comes to mind. It is done to non-reproductive cells and can’t be inherited by children.

The type of gene editing He did is called germline cell editing, also called heritable human gene editing (HHGE). It’s the editing of reproductive cells (sperm and egg cells) to create genetically modified embryos. The significance of this type of gene editing is that DNA alterations can be inherited by future generations because they extend to reproductive cells. Any changes made permanently affect the 'germline' of the person. Right now, HHGE is banned in most countries because of He's experiment, which broke the barrier between somatic and germline human gene editing.

A major reason that HHGE is controversial is its connection to the concept of eugenics, and the harmful history that comes with it. The ability to permanently alter DNA makes it possible to remove certain characteristics, which connects to the negative concept of eugenics. Eugenics is the theory of enhancing human populations by controlling breeding and inherited traits. It has been used to extremely negative, harmful, and discriminatory effects throughout the world, but especially in Nazi Germany. While it is debated, many stakeholders state that HHGE can be categorized as a eugenic practice because it is used to control inherited traits to a degree never before possible. This makes its potential very dangerous, and is why many people are wary of legalizing a tool that can edit the human germline.

However, there are different perspectives on the issue of HHGE and eugenics. As a medical technology, HHGE presents a way to treat genetic diseases that currently have no cure, and to prevent future people from being hurt by those diseases. If something harmful runs in a germline, HHGE technology can create genetic changes to improve health short-term and long-term through generations. For example, HHGE might be able to eliminate illnesses such as cystic fibrosis, muscular dystrophy, and sickle-cell anemia. But should HHGE use for disease/medical condition prevention be allowed? Would it be worth it to edit these harmful diseases from the germline, considering the potential for eugenics? Scientists have not come to a consensus.