1.    Introduction

If someone told you that the sex of an organism could be completely changed by altering a single letter in DNA — without a Y chromosome, without the SRY gene, without any intervention on the known genes that control this process — you would be skeptical. Reasonably so.

Yet that is exactly what a study published in April 2026 in Nature Communications demonstrates, conducted by researchers at Bar-Ilan University in Israel. Mice with two X chromosomes — genetically female — were born with a completely male body. Not partially, not ambiguously. Completely. The cause: the insertion of a single nucleotide (the smallest unit of DNA, equivalent to a single letter in a code of billions of characters) into a region of DNA that does not even encode a protein.

Until this discovery, biologists understood the female genetic program to be robust — that destabilizing it required eliminating several factors simultaneously. A single change was not enough. This study shows they were wrong — or more precisely, that they had not known where to look.

To understand why this discovery changes something fundamental, you need to know how biological sex determination works — and what researchers thought they knew about it.

2.     How Biological Sex Is Determined — What We Knew Before This Study

2.1 The Pro-Male Program: SRY and SOX9

In mammals, biological sex is not determined directly by the X and Y chromosomes — but by a cascade of molecular signals triggered by the genes they carry. It all begins with a single gene on the Y chromosome, called SRY. Its role is simple and precise: it activates another gene, Sox9.

Sox9 produces a protein — SOX9 — that functions as a master switch. Once activated, SOX9 triggers a chain of reactions that transforms the bipotential gonad (the embryonic structure that can become either an ovary or a testis) into a testis. Without SRY or SOX9, an XY individual is born with a female phenotype, regardless of chromosomes. With SOX9 forcibly expressed, an XX individual develops testes — demonstrated experimentally in both mice and humans.

2.2 The Pro-Female Program and Its Apparent Redundancy

Ovarian development depends on several pro-female factors acting together: RUNX1, FOXL2, WNT4, RSPO1, and others. Unlike the male program — where the loss of a single factor produces complete sex reversal — the female program appeared far more robust. Eliminating any individual pro-female factor produced only partial reversal. Two or more had to be removed simultaneously to achieve a significant effect. This suggested that pro-female factors compensate for each other — that the system has redundancy built into it.

2.3 Enh13 — The Enhancer That Controls Sox9

An enhancer (a regulatory element — a DNA sequence that does not encode any protein, but controls when and how strongly a nearby gene is activated) called Enh13 had been previously identified by the same research group as an essential regulator of Sox9 in the testis. Enh13 spans only 557 base pairs (units of DNA) and is located 565,000 base pairs away from the Sox9 gene — an enormous distance at the molecular scale, yet functionally irrelevant. Its deletion produces complete XY-to-female sex reversal. It was considered a strictly testicular element — active in the testis, irrelevant in the ovary. The 2026 study shows that conclusion was incomplete.

3. The Central Discovery — A Small Mutation, A Dramatic Effect

3.1 The 3 bp Deletion and 1 bp Insertion — The Adult Phenotype

The researchers generated, using CRISPR-Cas9 (a precision gene-editing technique that allows specific DNA sequences to be modified, similar to the find and replace function in a text editor), mice carrying tiny mutations inside Enh13 — specifically within the SOX9 binding site (the region of the enhancer where the SOX9 protein attaches to exert its effect).

The first mutation: a deletion (one or more letters missing from the DNA code) of 3 base pairs — three letters removed from Enh13's 557. The second: an insertion of a single base pair — one letter added in the same location.

The result was identical in both cases. XX homozygous mice (carrying the mutation on both copies of the gene) were born with male external genitalia and small testes. At the molecular level, the gonads of these mice no longer expressed ovarian markers and instead expressed testicular markers — at levels comparable to normal XY mice. The female program had been completely overwritten, without any Y chromosome, without the SRY gene.

One important detail from the study: heterozygous mice (carrying the mutation on only one copy) were normal — females with ovaries. The effect only appears when both copies carry the mutation. And XY mice with the same mutations developed normally as males — the mutations do not disrupt the male program.

3.2 The Embryonic Ovotestis — What Happens Before Complete Reversal

At early embryonic stages — before the gonad had fully committed — the picture was more complex. At embryonic day 13.5, the gonads of XX mutant mice were neither ovary nor testis, but an ovotestis (a gonad with mixed regions — ovarian and testicular portions coexisting within the same organ).

The variability was striking — some gonads appeared predominantly testicular, others had distinct ovarian and testicular portions. Even the left and right gonads of the same embryo could look different. This variability did not stem from genetic differences — all mice shared the same genetic background. It arose from the system's extreme sensitivity to the SOX9 activation threshold.

By adulthood, the ovotestis resolved completely into a testis. Mice with the single base pair insertion showed faster and more complete reversal than those with the 3 bp deletion — a detail that will make more sense in the section on mechanism.

Transcriptomic analysis (the study of all genes active simultaneously in a tissue) at embryonic day 12.5 confirmed the ovotestis picture at the molecular level — the XX mutant gonad simultaneously expressed genes specific to the testis and genes specific to the ovary, at intermediate levels compared to a purely male or female gonad.

4.     The Mechanism — Why It Works

4.1 Not a Stronger SOX9 Binding Site — The Hypothesis Ruled Out

The researchers' first hypothesis was logical: the mutations lie exactly where SOX9 binds to Enh13. Perhaps the change had created a stronger binding site — one that attracts SOX9 more efficiently and drives Sox9 activation more powerfully even without SRY.

They tested this hypothesis using two independent methods — protein binding microarray (a technique that measures how strongly a protein binds to different DNA sequences) and EMSA (electromobility shift assay — a method that directly visualizes whether and how strongly a protein binds to a DNA sequence). The result was clear: SOX9 binds to mutant Enh13 with equal or lower affinity than to normal Enh13. The mutations had not created a stronger SOX9 binding site. The hypothesis was ruled out.

4.2 RUNX1 Represses Enh13 in the Ovary — An Unexpected Discovery

If the mechanism was not a stronger SOX9, researchers asked what else had changed. They analyzed all binding sites within Enh13 and found something surprising: Enh13 contains binding sites not only for pro-male factors, but also for pro-female factors — including RUNX1. Moreover, the RUNX1 binding site partially overlaps with the SOX9 binding site.

Luciferase assays (tests in which a luminescent reporter gene shows how activated an enhancer is depending on which transcription factors are present) demonstrated that RUNX1 represses Enh13 activity — reducing it twofold in the absence of SOX9 and fourfold in its presence combined with other pro-female factors. This was a completely unknown function of Enh13. It was not merely an activator of Sox9 in the testis — it was also a repressor of Sox9 in the ovary, through RUNX1.

4.3 DNA Geometry Changes Everything

The final mechanism is the most subtle and most elegant finding in the study.

The researchers observed that the mutated region is highly conserved across mammals — not just the sequence, but the exact spacing between the binding sites of transcription factors (proteins that attach to DNA and control gene activation). The NR5A1 binding site lies 24 base pairs from the SOX9 site, the RUNX1 site partially overlaps with SOX9, and the GATA4 site sits 6 base pairs away. This precise spatial organization is conserved across all examined mammalian species — varying by no more than one nucleotide.

The extreme conservation of these distances suggests they are not accidental. The normal distance between the RUNX1 and GATA4 binding sites likely serves precisely to prevent physical contact between the two proteins.

The mutations alter this geometry. The 3 bp deletion shortens the distance between the RUNX1 and GATA4 binding sites by roughly one third of a helical turn of DNA (DNA has a spiral structure — one complete turn spans approximately 10 base pairs). This shortening may be sufficient to place the two proteins face to face rather than back to back along the DNA spiral, facilitating physical interaction between them. When RUNX1 and GATA4 interact physically, the complex acquires activating rather than repressive capacity — enough to raise Sox9 above the threshold for self-amplification.

The 1 bp insertion acts differently but produces the same effect: it creates a de novo binding site (new, absent from the normal sequence) for GATA4, adjacent to the RUNX1 site. This additional site allows cooperative binding of two GATA4 molecules in close proximity to RUNX1 — amplifying activation.

In both cases, once Sox9 exceeds the minimal activation threshold, SOX9 takes over and self-amplifies — replacing RUNX1 on Enh13 and driving complete testicular development.

5.     The Link to Humans

The study does not stay with mice. In the Discussion, the researchers mention three human patients with a 46,XX karyotype — meaning two X chromosomes, no Y chromosome — who were born with a male phenotype. None of them carried the SRY gene. All three had instead a small duplication of the region containing the human equivalent of Enh13.

A duplication means that portion of DNA appears twice instead of once. The exact mechanism by which this duplication produced male development had never been experimentally resolved. The 2026 study offers for the first time a plausible explanation: an extra copy of Enh13 may titrate (consume, neutralize) inhibitory factors such as RUNX1, releasing the enhancer from repression and allowing Sox9 activation in the absence of SRY. The same principle demonstrated in mice through point mutations may occur in humans through duplication of the entire element.

The phenomenon is not limited to humans. Pigs also present cases of XX sex reversal in the absence of SRY. The cause had been mapped to the Sox9 locus, but sequencing of the Sox9 gene, its promoter, and the TESCO enhancer had not revealed the responsible variant. The study suggests that variants in the Enh13 equivalent in pigs may be involved — a research direction explicitly left open.

The three human cases and the porcine cases are not direct evidence — the study did not sequence Enh13 in these patients and did not demonstrate the mechanism experimentally in humans. They are, however, strong convergences suggesting that what was demonstrated in mice reflects a real biological mechanism present in other species, including humans.

6.     Why It Matters

The first concrete thing this study changes is medical diagnosis. There are people born with a biological sex that does not match their chromosomes, and no one knows why. They have no Y chromosome, no SRY gene, no mutations in any of the known genes involved in sex determination. Doctors find no cause. These patients live without a precise molecular diagnosis — without knowing what in their DNA produced this difference. The study shows the answer may lie in Enh13 or similar regulatory elements — regions of DNA that no one was systematically analyzing in such cases until now. Concretely: in the future, these patients may be correctly diagnosed by sequencing these regulatory elements, not just the known genes.

The second thing it changes is how we understand biological sex differences in the context of disease. Many conditions evolve differently in women and men — autoimmune, cardiovascular, neurological diseases. Some of these differences are explained by hormones, but not all. If biological sex determination depends on regulatory elements far more sensitive than we thought, subtle variations in these elements may produce subtle variations in sexual biology that can influence disease susceptibility. This is an open research direction, not a conclusion — but the study provides the molecular tools to explore it.

The third thing — and the broadest — is the principle it demonstrates. If a single letter in a region of DNA that produces no protein can completely change the sex of an organism, the same mechanism may operate in other biological processes. Organ development, immune response, cancer susceptibility — all are partly controlled by non-coding regulatory elements. By understanding exactly how a minor geometric change in an enhancer's architecture can overturn a developmental program, science gains a model for finding similar mechanisms anywhere else in biology.

7.Conclusion

The biology of sex determination seemed, until now, well understood. Two antagonistic genetic programs, a Y chromosome that triggers masculinization, a robust female program that resists destabilization. This study does not overturn everything we knew — but it shows we understood the mechanism incompletely.

Enh13 is not merely an activator of Sox9 in the testis. It is simultaneously a repressor of Sox9 in the ovary. The same element, with opposing functions, governed by the molecular geometry of DNA — by the exact spacing between the sites where regulatory proteins bind. A single letter changed in this architecture is sufficient to transform a repressor into an activator and completely alter the fate of an organism.

Beyond sexual biology, the lesson is broader: regions of DNA that produce no proteins are not decorative. They control when, where, and how strongly genes are activated — and their sensitivity to tiny changes is far greater than we thought.

How many other such switches exist in the human genome, governing how many other biological processes, remains to be discovered.

Original source: Abberbock E, Ridnik M, Stévant I et al. A single-nucleotide enhancer mutation overrides chromosomal sex to drive XX male development. Nature Communications, vol. 17, art. 3186, 2026. https://www.nature.com/articles/s41467-026-71328-9