At first glance, it looked like a genetic error — so severe it should have crippled the animal. But instead, it helped turn horses into nature’s long-distance runners.

Researchers studying the DNA of nearly 200 mammal species noticed something strange in the genome of horses: a mutation in a gene that should have shut down a key cellular process. The gene, KEAP1, is vital for defending cells from the chemical stress caused by high levels of oxygen metabolism. Yet somehow, horses weren’t just surviving with this broken gene — they were thriving.

A Deal With the Devil, Perfected

“To make energy, we’ve made a deal with the devil,” Gianni Castiglione, an evolutionary biologist at Vanderbilt University, told NPR. “To, basically, have a slow burning fire in our cells.”

That fire powers our lives, turning oxygen into energy in the form of ATP. But it also produces reactive oxygen species (ROS), toxic molecules that can damage DNA, proteins, and cell membranes. For most animals, this presents a trade-off: make too much energy, and cells get overwhelmed by damage. But horses, it seems, found a workaround.

“Horses can make this fire burn even hotter and make the damage even less than it would be in a species like a human,” Castiglione said.

Published in Science, the new study reveals how a rare genetic mutation in the KEAP1 gene allows horse cells to turbocharge their energy production while also ramping up their built-in stress defenses. This ability may explain how horses evolved from dog-sized forest dwellers into muscular athletes that dominate endurance racing, cross deserts, and escape predators with ease.

Breaking the Genetic Rules

The KEAP1 gene normally codes for a protein that acts like a molecular bouncer. It binds to another protein, NRF2, keeping it out of the cell’s control room — the nucleus — where it could activate stress-response genes.

When ROS levels spike, the KEAP1 protein lets go of NRF2, allowing it to enter the nucleus and trigger antioxidant defenses. It’s a delicate balance that, if disrupted, can lead to inflammation or cell death.

In horse DNA, researchers found what looked like a catastrophic error: a premature stop codon — a three-letter genetic sequence (UGA) that tells the cell’s protein-making machinery to stop translating the gene.

“That kind of mutation kills mice,” Castiglione said. “We thought, wow, how are horses dealing with this?”

In most cases, a stop codon near the beginning of a gene would mean no functional protein gets made. It’s like halting the assembly of a car after installing just one bolt. The KEAP1 gene codes for a protein more than 600 amino acids long; the mutation in horses should have left it with only 14.

But when Castiglione and his colleagues examined horse cells, they found something astonishing: KEAP1 was still being made — fully formed and functional.

“Then one day, a light bulb went off,” Castiglione recalled in an interview with Science.

Instead of obeying the stop signal, the horses’ cells ignored it. They recoded the UGA stop codon as cysteine — an amino acid with special chemical properties. This rare phenomenon, called stop codon readthrough, had been seen mostly in viruses. In horses, it was operating at the very heart of a critical stress-regulation system.

Turning a Genetic Bug into a Feature

“It should have caused a catastrophic loss of function for this protein,” said Samantha Brooks, a horse genetics expert at the University of Florida. “But instead, the ancestors of these species somehow managed to really turn that lemon into lemonade.”

The genetic workaround doesn’t just keep the system running — it supercharges it.

Adding an extra cysteine to the KEAP1 protein makes it more sensitive to oxidative stress. That allows horse cells to respond faster and more effectively to ROS. Metabolic assays showed that horse muscle cells produce five times more ATP than mouse cells, while simultaneously increasing their antioxidant response by 200%.

“This provides them with the biochemical means to run fast and over long distances,” said Elia Duh, a clinician-scientist at Johns Hopkins University and co-lead author of the study.

The scientists used CRISPR, mass spectrometry, and metabolic assays on both horse and human cells. They found two proteins — known as SBP2 and eEFSec — that carry unique mutations to the horse genus (Equus). These proteins seem to have evolved in parallel with changes in the KEAP1 gene itself.

Together, the gene mutation and its supporting cast act like a cellular GPS that reroutes traffic around a roadblock.

Evolutionary and Biomedical Implications

This unusual mutation appears in all living members of Equus, including donkeys and zebras. That suggests the change occurred early in the lineage and may have played a foundational role in the evolution of the group’s remarkable endurance.

“Once they figured out how to run, they could occupy all kinds of ecological niches,” Castiglione said.

José Calbet, an exercise physiologist not involved in the study, called the work “exceptional,” praising its detailed molecular approach to a long-standing evolutionary puzzle.

Beyond evolutionary insights, the findings could also inform medicine.

Roughly 10% of human genetic diseases are caused by premature stop codons that truncate important proteins. If researchers can figure out how horses recode such stop signals, it might offer new gene therapy approaches for conditions like cystic fibrosis or muscular dystrophy.

“The identification of this evolutionarily significant UGA recoding event represents a potentially seminal finding,” said Hozumi Motohashi, a biologist at Tohoku University.

For Duh, the implications go beyond horses.

“By looking at what evolution has figured out, we know this is a viable strategy,” he said. “Nature is showing us a path we might one day use to heal.”