ACTN3: Your Fast-Twitch Muscle Gene
One stop codon decides whether your fast muscle fibers are built for the sprint or the long haul.
ACTN3 makes a protein called alpha-actinin-3 that sits in your fast-twitch muscle fibers, the ones you fire for a sprint or a heavy lift. A single common variant, R577X, can switch the gene off entirely. Roughly one in six people make none of the protein at all and feel perfectly fine, because a sibling protein covers the structural job. What changes is the metabolism underneath. This is the gene behind the "speed gene" headlines, and the actual biology is more interesting than the headlines let on.
What ACTN3 does
Builds alpha-actinin-3, a scaffolding protein that anchors the contractile filaments inside fast-twitch (type 2) muscle fibers.
Tunes how those fast fibers make energy. With the protein present, fibers lean on quick anaerobic, glycogen-burning metabolism built for short powerful bursts. Without it, they shift toward slower, more efficient aerobic metabolism.
The R577X variant (rs1815739) turns an arginine codon into a stop codon. Two copies of the stop version means zero functional protein, a state shared by over a billion people.
Does not break your muscles. A related protein, alpha-actinin-2, handles the structural anchoring either way, so the difference shows up in performance style and fatigue resistance, not in damage.
Your variants, decoded
This is the only ACTN3 variant that matters for the power-versus-endurance story. The letter you carry tells you whether your fast-twitch fibers contain alpha-actinin-3. Heads up on reading your raw data: dbSNP records this SNP on the minus strand, so a 23andMe export may print the complementary letters (G/A instead of C/T). If you see G/A, read G where this table says C and A where it says T.
| CC | Two working copies (the R/R genotype). Your fast-twitch fibers are fully loaded with alpha-actinin-3 and lean toward fast, powerful, glycolytic contractions. This is the most power- and sprint-associated version. |
| CT | One working copy, one switched off (R/X). You make alpha-actinin-3, just less of it. A middle-ground profile that sits between the two extremes. This is the most common genotype in most populations. |
| TT | Both copies switched off by R577X (X/X), so you make no alpha-actinin-3 at all. About 16% of people worldwide. Your fast fibers shift toward more efficient aerobic metabolism and resist fatigue a bit longer during sustained effort, which tilts the profile toward endurance. |
Genotypes are shown order-insensitively and on the forward strand; your own export may print the complementary letters — the meaning is the same.
What the research suggests
In a knockout mouse model, switching off alpha-actinin-3 (the state equivalent to human TT) shifted fast-twitch fibers toward efficient aerobic metabolism and away from the quick anaerobic, glycogen-burning system that powers short bursts. People who keep the protein (CC, and to a lesser degree CT) run more on that fast anaerobic system, which is exactly the phosphocreatine pathway creatine tops up. So if your fast fibers are the power-leaning kind, creatine is a sensible, well-studied support for short high-intensity efforts. It is supportive, not corrective, and useful for plenty of TT people too once they train for power.
PubMed 17828264 · knockout of alpha-actinin-3 in mice shifted fast-twitch muscle metabolism toward the more efficient aerobic pathway and raised intrinsic endurance, showing the protein's presence keeps fast fibers tilted toward anaerobic, power-style energy use.
Educational only — not medical advice. “General evidence” means the finding is real but the supplement’s benefit isn’t unique to your genotype.
See this matched to your own DNA — free.
Upload your 23andMe or AncestryDNA file and get your actual ACTN3 result, plus every other actionable variant — each line cited, your file never stored.
Questions
Does the TT genotype mean I'm bad at strength training?
No. TT (no alpha-actinin-3) is linked on average with a more endurance-leaning fast-fiber profile, but it's an average across groups, not a verdict on you. The metabolic shift toward aerobic energy is real, yet TT people build strength and muscle perfectly well with normal resistance training. Your genotype sets a tendency, not a ceiling.
Why do people call ACTN3 the 'speed gene'?
Because early studies found the CC (full-protein) version was overrepresented in elite sprinters and power athletes, while the TT version showed up more in some endurance athletes. The nickname stuck. It oversells a single gene's role, though. Performance comes from hundreds of genes plus years of training, and ACTN3 is one modest input among many.
My 23andMe file shows G and A, not C and T. Did I get the wrong gene?
Probably not. This SNP is recorded on the minus strand in dbSNP, so consumer reports sometimes print the complementary letters. G corresponds to the C in our table and A corresponds to the T. If you're unsure, check the rsid (rs1815739) matches and read the alleles by that mapping.
Should I change my supplements based on ACTN3 alone?
It's a weak basis for big changes on its own. ACTN3 describes a metabolic tendency in your fast fibers, not a deficiency you need to correct. Anything you'd consider, like creatine for power work, is decided mostly by your training and goals, not one genotype. Treat this page as education, not a prescription.