ACTN3 rs1815739: the 'sprint gene'

Genetic marker guide · Updated June 2026

23andMe & raw DNA

What ACTN3 does

The ACTN3 gene carries the instructions for a protein called alpha-actinin-3. This protein is part of the structural scaffolding inside muscle cells — specifically, it sits at the "Z-line," the anchor point that holds the contracting machinery of a muscle fiber in place and helps it transmit force efficiently.

What makes alpha-actinin-3 interesting is where it lives. It is found almost exclusively in fast-twitch (type II) muscle fibers — the fibers built for short, explosive, high-force efforts. It is essentially absent from slow-twitch fibers.

To understand why that matters, it helps to know the two broad families of muscle fiber:

• Fast-twitch (type II) fibers contract quickly and forcefully but fatigue relatively fast. They power sprinting, jumping, throwing, and heavy lifting.
• Slow-twitch (type I) fibers contract more slowly, produce less peak force, but resist fatigue and rely heavily on oxygen. They power sustained efforts like distance running and cycling.

Everyone has a mix of both, and the proportions vary from person to person and from muscle to muscle. Because alpha-actinin-3 is specialised to the fast-twitch fibers, researchers have long been curious about whether the gene that makes it influences how those explosive fibers behave. That curiosity is where the "sprint gene" nickname comes from — but as we'll see, the story is far more modest than the nickname suggests.

The R577X variant and the three genotypes

The variant reported at rs1815739 is the same one geneticists have historically called R577X. The two names describe one change in the ACTN3 gene:

• C allele (R577): the functional version. The gene reads through normally and produces alpha-actinin-3.
• T allele (X577): a premature stop codon. It ends the protein early, so this copy of the gene produces no functional alpha-actinin-3 at all.

Because you inherit one copy from each parent, that gives three possible genotypes:

A substantial fraction of people worldwide are TT and produce no functional alpha-actinin-3 whatsoever — and they are perfectly healthy. This is one of the most reassuring facts about ACTN3: a "missing" protein here is a normal human variant, not a deficiency, disease, or defect. The frequency of the T allele also varies considerably between populations with different ancestries, which is one reason single-gene "athletic" claims travel so poorly from one group to another.

Why "no ACTN3" isn't a problem: alpha-actinin-2 compensates

If alpha-actinin-3 is important for fast-twitch fibers, how can so many people function normally — including elite athletes — with none of it?

The answer is a close relative: alpha-actinin-2 (made by the ACTN2 gene). Alpha-actinin-2 is present in the same muscle fibers, and in people who lack alpha-actinin-3, it appears to step in and cover much of the same structural role. The compensation isn't perfectly one-to-one — research in this area suggests the two proteins aren't completely interchangeable, and losing alpha-actinin-3 seems to shift fast-twitch fibers toward slightly more endurance-like, fatigue-resistant properties. But the practical upshot is clear: having the TT genotype does not leave you with weak or broken muscles. It is a subtle biochemical tilt, not a handicap.

What research has — and hasn't — shown

The reason ACTN3 became famous is a genuine research finding: when scientists compared groups of elite athletes to the general population, they saw statistical enrichment of certain genotypes. The functional (CC) genotype tends to be more common among top-level sprint and power athletes, and the non-functional (TT) genotype tends to be more common among top-level endurance athletes. That pattern has been reported across multiple studies and populations, and it is a real, if debated, signal.

Here is the crucial distinction, and it's the whole ballgame:

• Population level: across large groups of elite athletes, genotype frequencies differ in a direction that fits the biology. This is a real association.
• Individual level: for any single person, ACTN3 is a weak predictor. Knowing your genotype tells you very little about how fast, strong, or fit you specifically can become.

Those two statements are not in conflict. A gene can nudge the odds across thousands of people while being almost useless for forecasting one person's outcome — the same way knowing a population's average height tells you nothing reliable about the next person to walk through the door. The effect attributable to ACTN3 is modest even in the studies that find it, and it sits inside an enormous web of other influences.

It's also worth being honest that the science is not settled. Some studies find the association, others find it weakly or not at all, and results depend heavily on the population studied, the sport, the definition of "elite," and sample size. Muscle performance is polygenic — shaped by many genes, each contributing a small amount — and ACTN3 is just one thread in that fabric.

Why it is not a talent predictor

Even setting the genetics aside, athletic performance is overwhelmingly the product of things that have nothing to do with any single variant:

• Training — years of consistent, well-structured work is the single largest factor.
• Environment and access — coaching, facilities, nutrition, recovery, and opportunity.
• Body structure — limb proportions, tendon properties, and overall build.
• Physiology beyond one gene — heart and lung capacity, and the combined effect of many other genes.
• Psychology — motivation, resilience, and the simple willingness to keep showing up.

You can find plenty of elite athletes competing — and winning — with the "wrong" ACTN3 genotype for their event. That alone should settle the question: the gene is not destiny. It's a small, interesting nudge in a system dominated by effort and circumstance. If you're curious about your own leanings, the most reliable evidence isn't a DNA letter — it's how your body actually responds to training, which you can only learn by training.

The direct-to-consumer "sports gene" testing caveat

The gap between the real science and the marketing is why "sports gene" tests deserve real skepticism. A number of companies sell tests — sometimes marketed at parents of young children — that promise to reveal a child's ideal sport, their "power vs endurance" destiny, or their untapped athletic potential, often built around ACTN3.

The evidence does not support these claims. Major sports-science and genetics bodies have cautioned that using single-gene tests to steer talent identification, or to push a child toward or away from a sport, is not scientifically justified and may cause harm — discouraging a kid who would have loved and excelled at an activity, or narrowing a young person's experience based on a marker that predicts almost nothing about them individually. A responsible reading of ACTN3 is educational, not prescriptive: it is not a scouting tool, and it should never be used to make decisions about a person's future.

What a curious person can reasonably take from it

None of this means ACTN3 is uninteresting. Quite the opposite — it's one of the clearest, most accessible examples of genotype-to-biology you'll find in a consumer DNA file. It shows how a single letter can switch a specific protein on or off, how the body compensates when a protein goes missing, and how a real population-level association can still be a poor individual predictor. That's a genuinely useful lesson in how to read all genetic information wisely.

So a reasonable takeaway looks like this:

• Treat your genotype as a fun fact, not a training plan. It's context, not a coaching instruction.
• Let it teach you the difference between "associated with" and "determines." Almost every marker in your file works this way.
• Don't let it discourage or over-encourage you. Your response to training is what actually matters, and you can't read it off a chromosome.

If you'd like to look yours up, here's how.

How to find rs1815739 in your raw data

1. Download your raw data (or from AncestryDNA / MyHeritage).
2. Search it for rs1815739 and read your two-letter genotype.
3. Or use our free DNA explorer — it reads your file in your browser, and nothing is uploaded.

Strand note: 23andMe usually reports rs1815739 on the C/T strand (C = functional, T = stop). If a tool or export shows the marker on the opposite strand, the letters will be complemented (G/A instead of C/T) — the underlying genotype is the same, just written against the other strand.

The bottom line

ACTN3 rs1815739 is a real and genuinely interesting variant: it decides whether your fast-twitch muscle fibers carry alpha-actinin-3, and its genotypes show up at different frequencies in elite sprint versus endurance athletes. But at the level of an individual person, it explains very little. Training and environment dominate, alpha-actinin-2 quietly covers for the missing protein, and the science remains modest and debated. Enjoy your result as a window into your own biology — not as a label, a limit, or a plan.

For everything else hiding in your file, see our complete guide to analyzing 23andMe raw data, or browse the rest of the Quanome blog.

Frequently asked questions

What does ACTN3 rs1815739 tell you?

ACTN3 codes for alpha-actinin-3, a structural protein found in fast-twitch muscle fibers. The rs1815739 variant determines whether you produce it: the functional allele makes the protein, while the other allele is a premature stop codon that produces none. It is a real, well-studied genotype-to-protein link — but it is one input among many, not a measure of athletic ability.

Which ACTN3 genotype is the 'sprinter' type?

The genotype with two functional copies is over-represented in elite sprinters and power athletes at the population level, while the genotype with no functional copies is over-represented in elite endurance athletes. The heterozygous genotype produces one working copy. These are statistical tendencies across large groups, not a verdict for any one person.

How do I find ACTN3 rs1815739 in my raw data?

Search your raw DNA file for rs1815739 and read the two-letter genotype, or use a tool that looks it up for you. 23andMe typically reports this marker on the C/T strand, where C is the functional allele and T is the stop codon.

Does ACTN3 determine athletic ability?

No. It is associated with a modest tendency toward power versus endurance at the population level, but training, environment, body type, motivation, sleep, coaching, and many other genes matter far more. Plenty of elite athletes have the 'wrong' genotype for their event. A raw-data result is not a talent test.

What is the R577X variant?

R577X is the classic name for the same variant reported at rs1815739. 'R577' is the functional version (arginine at position 577 of the protein), and 'X577' is the stop codon that ends the protein early and produces no functional alpha-actinin-3. So R577X and rs1815739 describe the same change.

If I have no functional ACTN3, is something wrong with me?

No. A large share of the world's population produces no functional alpha-actinin-3 and are entirely healthy. A closely related protein, alpha-actinin-2, is present in the same fibers and appears to compensate. This is normal human variation, not a deficiency or disease.

Should I choose a sport based on my ACTN3 result?

No. Direct-to-consumer 'sports gene' tests that promise to reveal a child's ideal sport are not supported by the evidence. A single gene cannot predict who will excel, and steering a person toward or away from a sport based on one marker risks doing more harm than good. Enjoyment, access, and consistent training are far better guides.

Is 23andMe raw data a performance or diagnostic test?

No. Raw genotyping data is not a diagnostic, medical, or performance test. It reports which letters you carry at specific positions; it does not measure your muscle, fitness, or potential. Treat markers like ACTN3 as educational context, and rely on qualified professionals and actual training for real decisions.