Is there really a sprinter gene? What ACTN3 says about your muscles
About one in five people of European descent is missing a muscle protein entirely. Their bodies have never made a single molecule of it. They run, lift, play football and carry the shopping like everyone else, and almost none of them will ever find out.
The protein is alpha-actinin-3, the gene behind it is ACTN3, and in 2003 it earned a nickname that stuck: the sprinter gene. Here is what it really does, what your own result means, and how to find it in your raw DNA file in two minutes.
What does the sprinter gene do?
Your muscles mix two kinds of fibers. Slow-twitch fibers are the marathoners: they keep working for hours without tiring. Fast-twitch fibers are the sprinters: they fire hard and fast for a few seconds, for a jump, a sprint or the last rep of a heavy lift.
Inside every fiber, thin strands slide past each other to make the muscle contract. Alpha-actinin-3 works like a clip that holds those strands in place while fast-twitch fibers pull at full speed. ACTN3 is the recipe for that clip.
A common spelling change in the recipe, called R577X (marker rs1815739), adds an early stop sign. Inherit it from both parents and the clip is never made. That sounds like a problem, and it isn't: a sister protein, alpha-actinin-2, steps in, and the muscle works fine. The difference only shows at the far end of sport.
What do CC, CT and TT mean?
Your raw data file writes rs1815739 as two letters, one from each parent. C is the working version (researchers call it R) and T is the off version (X).
| Your file shows | Alpha-actinin-3 | What the studies show | In your Helisoma report |
|---|---|---|---|
| CC (RR) | Made from both copies | Over-represented among elite sprint and power athletes | Built for power |
| CT (RX) | Made from one copy | In between, no strong pull either way | All-rounder muscle |
| TT (XX) | None | Rarer among elite sprinters, as common among endurance athletes as among everyone else | Less of the power lean |
What the famous 2003 study found
At the very top of sport, ACTN3 does look like a sprinter gene. In 2003, Australian researchers checked the gene in 107 elite sprint and power athletes, 194 elite endurance athletes and 436 ordinary people. Here is how many in each group had TT:
TT was three times rarer among the sprinters than among everyone else, and not one female elite sprinter or sprint Olympian in the study had it. A 2024 analysis that pooled 25 studies and 14,541 people from 13 countries found the same pattern: the working C version turns up more often in power athletes than in endurance athletes or non-athletes.
Does it matter if you're not an Olympian?
A little. For everyday people, each T copy goes with a small shift away from sprint and power, more like a light breeze than a wall. Training, age, body size and hundreds of other genes push much harder. The same 2003 study found 6 TT athletes among its 107 elite sprinters and power athletes, so TT does not close the door on power sports.
The honest way to read your result: it hints at which kind of training may feel natural first. It does not choose your sport, and no serious geneticist would pick a child's sport by an ACTN3 test.
Is there an upside to TT?
Possibly, when it's cold. In a 2021 study, researchers put men in cold water and tracked their core temperature. The TT men held their temperature better than the CC men. Their muscles leaned toward slow-twitch features and tensed up instead of shivering hard, and that kept them warmer. The authors think this is why the T version became more common as early humans moved into colder climates, although other researchers think chance explains the spread just as well.
How do I find my result in my raw data?
- Download your raw file. Here is how for every test.
- Open it in any text editor and search for rs1815739.
- Read the letters at the end of the line:
- 23andMe:
rs1815739 11 66328095 CT - AncestryDNA puts the letters in two columns:
rs1815739 11 66328095 C T - MyHeritage writes them in quotes:
"rs1815739","11","66328095","CT"
- 23andMe:
- CC, CT or TT is your answer. Two dashes (
--) or0 0mean the chip could not read this spot for you.
Or just drop the file into our free analysis. Your browser reads it in about a minute, the file never leaves your device, and your full report shows your ACTN3 result as "Built for power", "All-rounder muscle" or "Less of the power lean", with the study behind it.
Should you train differently?
Train for the sport you love. Sprints, jumps and heavy lifts build power in anyone, and steady mileage builds endurance in anyone; training changes what your fibers can do far more than this gene changes what they start as. If you are CC, power work may reward you quickly. If you are TT, peak power is still very trainable, and nothing here points you toward or away from any sport. A timed sprint or a max lift tells you more than any gene, so when your training log and your genotype disagree, believe the log.
Sources
- Yang N, MacArthur DG, Gulbin JP, et al. ACTN3 genotype is associated with human elite athletic performance. American Journal of Human Genetics, 2003. PubMed 12879365
- El Ouali EM, Barthelemy B, Del Coso J, et al. A systematic review and meta-analysis of the association between ACTN3 R577X genotypes and performance in endurance versus power athletes and non-athletes. Sports Medicine Open, 2024. PubMed 38609671
- Wyckelsma VL, Venckunas T, Houweling PJ, et al. Loss of alpha-actinin-3 during human evolution provides superior cold resilience and muscle heat generation. American Journal of Human Genetics, 2021. PubMed 33600773
