Two copies of a gene called Sox9 sit in every mouse cell. In females, they stay silent and ovaries form. In males, they roar to life and testes develop. The difference between silence and activation — between one organ and another — comes down to a 557-letter stretch of DNA that doesn’t code for any protein at all.

That stretch is called Enh13, and researchers at Bar-Ilan University have just shown that changing a single letter within it is enough to override chromosomal sex determination entirely. Female mice with XX chromosomes carrying mutations in both copies of Enh13 developed testes and male genitalia, according to a study published today in Nature Communications.

This is the latest in a series of experiments from the lab of Nitzan Gonen that together reveal Enh13 as a molecular switch — one that integrates opposing signals and tips the balance between male and female development. The work underscores a broader lesson: the vast non-coding regions of the genome, long dismissed as filler, contain regulatory machinery of extraordinary precision.

The switch that goes both ways

Mammalian sex determination works through a cascade. The Sry gene on the Y chromosome activates Sox9, which then drives testis formation. For ovaries to develop, Sox9 must be kept off. Enh13, located 565,000 base pairs upstream of Sox9, is the control panel.

In 2018, Gonen’s team showed that deleting Enh13 entirely caused XY male mice to develop female organs. In 2024, the group demonstrated that smaller disruptions — deleting just the transcription factor binding sites within Enh13 — produced the same sex reversal. Now the researchers have flipped the experiment in the other direction, activating a male program in females.

Using CRISPR genome editing, the team introduced tiny mutations into the SOX9 binding site of Enh13: a one-base-pair insertion in some mice, a three-base-pair deletion in others. Both mutations caused XX mice to develop as males, with testes and male genitalia. The one-base insertion produced a more complete testis phenotype than the three-base deletion, the paper notes.

“This is a remarkable finding because such a tiny change — just one DNA letter out of ~2.8 billion — was enough to produce a dramatic developmental outcome,” Gonen said.

How one letter breaks repression

The mechanism turns out to be unexpectedly elegant. Enh13 doesn’t simply activate Sox9 in males. It also actively represses it in females.

Pro-female transcription factors — including RUNX1, NR5A1, and GATA4 — bind to Enh13 and keep Sox9 silent. The SOX9 binding site within Enh13 overlaps with a RUNX1 binding site, creating what the researchers describe as a molecular battleground. The mutations don’t stop SOX9 from binding. Instead, they prevent the repressive factors from doing their job. Without that repression, even the low baseline level of Sox9 present in female embryos crosses a critical threshold. Once Sox9 activates, it amplifies its own expression in a positive feedback loop, and the testicular program takes over.

Both copies of Enh13 must be mutated for complete sex reversal. Mice with only one modified copy still developed female organs — a sign of how tightly the system is calibrated.

From mice to the clinic

The implications extend beyond basic biology. The Enh13 region is also important in human sex determination. Around half of people with differences of sex development (DSD) — conditions affecting roughly 1 in 4,000 births worldwide — have no genetic diagnosis, according to Katie Ayers, a genetics researcher at the Murdoch Children’s Research Institute in Melbourne. Standard sequencing examines only protein-coding regions, which account for roughly 2 percent of the genome.

“Our findings show that it is not enough to look only at genes,” said Elisheva Abberbock, the PhD student leading the research. “Important disease-causing mutations may also lie in the non-coding genome, in DNA regions that control gene activity rather than encode proteins.”

Ayers noted that looking for small changes in the Enh13 region could help identify mutations responsible for DSD cases that currently go unexplained.

The regulatory genome, letter by letter

Enh13 is 557 base pairs long, sits hundreds of thousands of base pairs from its target gene, and integrates at least four different transcription factors to produce a binary output: testes or ovaries. That a single nucleotide can rewire this circuitry is striking — and it suggests that many other developmental decisions may be controlled by similarly compact regulatory elements hiding in the non-coding genome.

Gonen’s team is now working to identify additional regulatory regions involved in sex determination and to test their function systematically. Enh13, it seems, is a beginning.

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