Sex Development, Explained?

Every year, about 1 in 4,000 babies is born with a Difference of Sex Development (DSD), a condition where biological sex doesn’t follow typical patterns.

For many families, one question quickly follows: How did this happen?

Doctors look at the genes. They run advanced genetic tests. They search for answers in the parts of DNA we’ve studied for decades.

And in more than half of these cases… they find nothing. No mutation. No explanation.

Now, researchers at Bar-Ilan University believe they may have found what’s been hiding in that space all along.

The answer wasn’t in the genes

In a new study published in Nature Communications, a team led by Dr. Nitzan Gonen discovered something surprising:

A change in just one letter of DNA, outside of any gene, was enough to completely change how sex develops.

In their experiment, mice with XX chromosomes, which normally develop as females, developed instead as males, with full male anatomy.

The part of DNA we’ve been overlooking

Most people think of DNA as a set of genes, instructions that build the body.

But genes are only about 2% of the story.

The other 98% doesn’t build anything directly, but it controls timing, decides which genes turn on, when, and how strongly.

For years, this part of DNA was overlooked.

This study shows why that may have been a mistake.

A switch that changes everything

The researchers focused on a tiny stretch of DNA called Enh13.

Its job is to control a key gene called Sox9, which drives the development of testes.

You can think of it simply:

If Sox9 turns on → male development
If Sox9 stays off → female development

Enh13 is the switch that helps decide.

In female development, this switch keeps Sox9 turned off.

But when the researchers made a tiny change, adding just one DNA letter, that “off” signal stopped working.

Sox9 turned on.

And the body followed.

A missing piece finally revealed

This is what makes the discovery so powerful.

It shows that the reason many DSD cases remain unexplained may be because we’ve been looking in the wrong place.

While we’ve been searching inside genes, the real cause may sit just outside them.

As PhD researcher Elisheva Abberbock explains, “focusing only on genes can mean missing the mutations that actually drive the outcome”.

What this changes

This discovery changes how we think about genetic diagnosis, development, and even disease.

If a single “letter” in a regulatory region can reshape something as fundamental as sex development, then many other conditions may also trace back to these hidden parts of DNA.

The research team is now working to map these regions more broadly, hoping to uncover more answers that until now have remained invisible.

For families still searching for explanations, this kind of discovery does something powerful.

It turns “we don’t know why” into “we may finally know where to look.”