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Guide · Gene explainer

Androgen receptor CAG repeat test: what it measures and what it can’t tell you

The androgen receptor is the protein testosterone works through. A stretch of its gene repeats the letters CAG, and the number of repeats varies between men. Here is what that number has been linked to, how strong the evidence is, and how it is tested in the UK.

Quick answer

An androgen receptor CAG repeat test counts the CAG repeats in the AR gene, usually between about 10 and 32 in men. Shorter repeats make the receptor more active in the laboratory, but in people the effects are small, and no study has looked at the repeat in anabolic steroid users.

  • Measured by fragment analysis or sequencing. Consumer chips like 23andMe's can't read it.
  • On the NHS it is tested when Kennedy disease is suspected, which needs a specialist referral.
  • The largest study (181,217 men) found circulating testosterone matters more than repeat length.
Gene
AR, the androgen receptor, on the X chromosome. Men have one copy, so one repeat length.
Usual range
About 10 to 32 repeats in young men; averages differ by ancestry.
In the lab
Longer repeats make the receptor less active, roughly in proportion to length.
In people
Small effects on testosterone, prostate growth on therapy, bone density and baldness.
Tested by
Fragment analysis or DNA sequencing. Not by genotyping chips.
Steroid users
Not studied.

What the CAG repeat is

Testosterone and DHT act by binding the androgen receptor, which then switches genes on and off in muscle, bone, skin, the prostate and the brain. The first part of the AR gene contains a run of CAG letters, which builds a chain of the amino acid glutamine into the receptor. Laboratory studies in 1994 showed that lengthening the run lowers the receptor’s activity roughly in step with its length, and removing it raises activity.[1]

In 767 Danish men aged 20 to 29, repeat lengths ranged from 10 to 32.[2] Averages differ by ancestry: in a Russian study the median was 23 in Slavic men and 24 to 25 in Buryat and Yakut men.[3] The AR gene is on the X chromosome, so a man inherits his one copy from his mother.

What studies have found

OutcomeWhat was foundEvidence
Testosterone levelLonger repeats went with slightly higher total testosterone in 181,217 UK Biobank men, as if the body compensates for a less active receptor.[4]Well replicated
Prostate on testosterone therapyIn 131 men treated for low testosterone, the prostate grew more in men with shorter repeats; under 20 repeats, the odds of a prostate over 30 ml were 8.7 times higher.[5]Some evidence
Response to testosterone therapyMixed. A Korean study of 42 men found no link between repeat length and the effect or safety of therapy.[6]Early research
Male-pattern baldnessLonger repeats went with slightly less baldness in UK Biobank.[4]Some evidence
Anabolic steroid response or side effectsNo studies found.Early research

Why the effect is smaller than it sounds

The body adjusts. In the European Male Ageing Study of nearly 2,900 men, longer repeats went with higher testosterone and oestradiol, which the authors put down to more testosterone being converted to oestradiol rather than to weaker androgen action.[7] The UK Biobank study, which measured the repeat in 181,217 men from their genome data, found the effects were small, that the repeat wasn’t linked to most outcomes, and that circulating testosterone is a more important driver of androgen action than repeat length.[4]

What it can’t tell you

Your CAG repeat can’t tell you how you will respond to anabolic steroids, whether they are safe for you, or what to take. It has not been studied in steroid users at all.

How it is tested

The repeat is counted by fragment analysis (capillary electrophoresis) or read from DNA sequencing.[3, 4] Genotyping chips, including those used by 23andMe and AncestryDNA, read single-letter variants and don’t readily capture repeat lengths.[8] You can still check other studied hormone-gene variants in your raw data.

Getting tested in the UK

  • NHS: the CAG repeat is tested when a specialist suspects Kennedy disease (spinal and bulbar muscular atrophy), which is caused by a much longer repeat, around 38 or more.[9] It needs a referral, usually from neurology.
  • Private, direct to the public: we found no UK laboratory offering it in October 2026. Marek Diagnostics sells a saliva test for $500, apparently to US customers only.
  • Broader UK DNA tests: some list the AR gene among others without saying whether they count the repeat; ask before ordering. See the UK DNA tests compared.

Questions

What is an androgen receptor CAG repeat test?

It counts how many times the three letters CAG repeat in a stretch of the androgen receptor (AR) gene. The count varies between men, usually from about 10 to the low 30s. It is measured by fragment analysis or DNA sequencing, not by the chips used in consumer ancestry tests.

Can I get an androgen receptor CAG repeat test in the UK?

On the NHS, the CAG repeat is tested when a specialist suspects Kennedy disease (spinal and bulbar muscular atrophy), which needs a referral. We found no UK laboratory selling it directly to the public as of October 2026. Marek Diagnostics sells one for $500, but appears to sell in the US only.

Does a short CAG repeat mean I respond better to testosterone?

Not in any way you could rely on. In the laboratory, shorter repeats make the receptor somewhat more active. In people, the largest study, of 181,217 men in UK Biobank, found only small effects, and concluded that circulating testosterone matters more than repeat length.

Has CAG repeat length been studied in anabolic steroid users?

We found no study of CAG repeat length in people who use anabolic steroids. The evidence comes from men's natural hormones and from men on prescribed testosterone, so it can't be used to predict how someone will respond to steroids.

Can 23andMe or AncestryDNA tell me my CAG repeats?

No. Their genotyping chips read single-letter variants, not the length of repeats. You can check other studied hormone-gene variants in their raw data with our free checker.

Is a long CAG repeat a disease?

Repeats in the usual range are normal variation. A much longer repeat, around 38 or more, causes Kennedy disease, a rare inherited condition affecting nerves and muscles that is diagnosed by NHS genetic services.

Sources

  1. Chamberlain NL, Driver ED, Miesfeld RL. The length and location of CAG trinucleotide repeats in the androgen receptor N-terminal domain affect transactivation function. Nucleic Acids Res 1994;22:3181–86.
  2. Nielsen TL et al. The Odense Androgen Study. Eur J Endocrinol 2010;162:795–804.
  3. Osadchuk LV et al. Vavilovskii Zhurnal Genet Selektsii 2024;28:80–89.
  4. Sasako T et al. The influence of trinucleotide repeats in the androgen receptor gene on androgen-related traits and diseases. J Clin Endocrinol Metab 2024;109:3234–44.
  5. Zitzmann M et al. Prostate volume and growth in testosterone-substituted hypogonadal men are dependent on the CAG repeat polymorphism of the androgen receptor gene. J Clin Endocrinol Metab 2003;88:2049–54.
  6. Translational Andrology and Urology: CAG repeats and testosterone therapy in Korean men.
  7. Huhtaniemi IT et al. Increased estrogen rather than decreased androgen action is associated with longer androgen receptor CAG repeats. J Clin Endocrinol Metab 2009;94:277–84.
  8. Saini S et al. A reference haplotype panel for genome-wide imputation of short tandem repeats. Nat Commun 2018;9:4397.
  9. NHS Genomics Education Programme. Spinal and bulbar muscular atrophy (Kennedy disease).

For information only, not medical advice. GeneticEdge does not recommend or endorse anabolic steroid use, and no genetic test can tell you whether steroids are safe for you. Talk to a GP or pharmacist about your health. Last reviewed 30 October 2026.