Neurology

Genetics and Diagnosis: Finding the Answer

At a Glance

Kennedy Disease is definitively diagnosed using a genetic blood test that checks for 38 or more CAG repeats in the androgen receptor (AR) gene. Because it is an X-linked genetic condition, it primarily affects men and progresses much slower than similar conditions like ALS.

Understanding the science behind Kennedy Disease is the first step in taking control of your health. Because this condition is rare and its symptoms develop slowly, it is often misdiagnosed as other conditions, like ALS (Amyotrophic Lateral Sclerosis). Knowing how the disease works biologically and how it is definitively diagnosed can help ensure you receive the right care ^[1]^[2].

The Genetic “Stutter”: CAG Repeats

At the heart of Kennedy Disease is a mutation in the androgen receptor (AR) gene ^[3]. This gene provides the instructions for building a receptor that allows your body to respond to male hormones (androgens) like testosterone.

Inside this gene is a sequence of three DNA building blocks—Cytosine, Adenine, and Guanine—known as a CAG repeat ^[4].

In most people, this sequence repeats about 10 to 36 times ^[4].
In people with Kennedy Disease, this sequence “stutters” and repeats 38 times or more ^[3].

This expansion causes the body to produce a protein with an extra-long segment. This “toxic” protein doesn’t fold correctly and builds up inside your motor neurons and muscle cells, eventually causing them to stop working ^[5]^[6].

Why It Primarily Affects Males

Kennedy Disease follows an X-linked recessive inheritance pattern ^[2].

Men have one X chromosome and one Y chromosome. If their only X chromosome has the mutation, they will develop the disease.
Women have two X chromosomes. If one has the mutation, the healthy second X chromosome usually “compensates,” meaning they typically do not show symptoms. These women are called carriers ^[7].

Furthermore, the toxic protein caused by the mutation is primarily activated by androgens (male hormones). Because men have much higher levels of these hormones than women, the protein becomes much more harmful in the male body ^[5]^[8].

Confirming the Diagnosis

The only way to definitively diagnose Kennedy Disease is through genetic testing ^[9]. A simple blood test can look specifically at the AR gene to count the number of CAG repeats.

While other tests can provide clues, they cannot replace genetic confirmation:

Electromyography (EMG): Measures the electrical activity of your muscles and nerves. In Kennedy Disease, it often shows damage to both motor and sensory nerves ^[10]^[11].
Blood Tests: May show high levels of creatine kinase (CK), a marker of muscle breakdown, or signs of insulin resistance ^[2]^[12].

Kennedy Disease vs. ALS

Because both conditions involve muscle weakness and “twitching” (fasciculations), they are often confused. However, there are key differences that help doctors tell them apart:

Feature	Kennedy Disease (SBMA)	ALS
Genetic Test	Positive for AR gene expansion ^[9]	Negative for AR gene expansion
Sensory Nerves	Often damaged (numbness) ^[10]	Usually healthy
Reflexes	Usually reduced or absent ^[9]	Often overactive or “brisk”
Hormonal Signs	Common (gynecomastia) ^[13]	Absent
Progression	Very slow (decades) ^[2]	Rapid (years)

The presence of gynecomastia (breast enlargement) is one of the most helpful clinical signs, as it is very common in Kennedy Disease but almost never seen in ALS ^[13]. If your doctor suspects a motor neuron disease, ensuring they consider Kennedy Disease—especially if you have tremors or hormonal changes—is vital for an accurate diagnosis ^[9]^[14].

Return to Overview

Common questions in this guide

How is Kennedy Disease diagnosed?

The only definitive way to diagnose Kennedy Disease is through a genetic blood test. This test checks the androgen receptor (AR) gene for an abnormally expanded number of CAG repeats.

What is a CAG repeat in Kennedy Disease?

A CAG repeat is a specific sequence of DNA building blocks in the androgen receptor gene. In people with Kennedy Disease, this sequence stutters and repeats 38 times or more, which causes the body to produce a harmful protein that damages motor neurons.

Why does Kennedy Disease primarily affect men?

Kennedy Disease is an X-linked condition, meaning the genetic mutation is carried on the X chromosome. Because men have only one X chromosome and higher levels of male hormones that activate the harmful protein, they develop symptoms, while women typically remain unaffected carriers.

How can doctors tell the difference between Kennedy Disease and ALS?

While both conditions cause muscle weakness and twitching, Kennedy Disease progresses much more slowly over decades. Kennedy Disease is also confirmed with a genetic test and often features sensory nerve damage, absent reflexes, and hormonal signs like breast enlargement, which are typically absent in ALS.

What does it mean for my family if I test positive for the AR gene mutation?

Because it is an inherited genetic condition, your diagnosis has direct implications for your blood relatives. Your daughters will be carriers of the gene, and female relatives like sisters may also carry the mutation, so consulting a genetic counselor is highly recommended.

Curated prompts to bring to your next appointment.

1.Can you explain the results of my genetic test and confirm the number of CAG repeats found?
2.Since this is an X-linked condition, what does this mean for my children and my sisters?
3.How do my physical exam findings, such as my reflexes and the presence of tremors, help you distinguish this from ALS?
4.Would a sensory nerve conduction study be helpful to confirm the diagnosis or monitor my progression?
5.If I have a female relative with mild muscle symptoms, should she also be tested for the AR gene mutation?

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References (14)

1
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Malek EG, Salameh JS, Makki A
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PMID: 32839928
2
Spinal and Bulbar Muscular Atrophy.
Grunseich C, Fischbeck KH
Neurologic clinics 2015; (33(4)):847-54.
PMID: 26515625
3
Correlation between the CAG repeat size and electrophysiological findings in patients with spinal and bulbar muscular atrophy.
Kim H, Lim YM, Lee EJ, et al.
Muscle & nerve 2018; (57(4)):683-686 doi:10.1002/mus.25977.
PMID: 28972672
4
Molecular pathogenesis of spinal bulbar muscular atrophy (Kennedy's disease) and avenues for treatment.
Grunseich C, Fischbeck KH
Current opinion in neurology 2020; (33(5)):629-634 doi:10.1097/WCO.0000000000000856.
PMID: 32773451
5
Spinal and Bulbar Muscular Atrophy Overview.
Fischbeck KH
Journal of molecular neuroscience : MN 2016; (58(3)):317-20 doi:10.1007/s12031-015-0674-7.
PMID: 26547319
6
A Crucial Role for the Protein Quality Control System in Motor Neuron Diseases.
Cristofani R, Crippa V, Cicardi ME, et al.
Frontiers in aging neuroscience 2020; (12()):191 doi:10.3389/fnagi.2020.00191.
PMID: 32792938
7
Kennedy disease in two sisters with biallelic CAG expansions of the androgen receptor gene.
Müller KI, Nilssen Ø, Nebuchenykh M, et al.
Neuromuscular disorders : NMD 2022; (32(1)):75-79 doi:10.1016/j.nmd.2021.11.007.
PMID: 34922802
8
Case report: Klinefelter syndrome may protect against the development of spinal and bulbar muscular atrophy.
Akanuma H, Kadowaki S, Kanai K
Frontiers in neurology 2024; (15()):1340694 doi:10.3389/fneur.2024.1340694.
PMID: 38405402
9
[Clinical features of a genetically identified spinal and  bulbar muscular atrophy pedigree].
Wang Z, Chen Q, Li Q, Bi F
Zhong nan da xue xue bao. Yi xue ban = Journal of Central South University. Medical sciences 2016; (41(10)):1101-1105 doi:10.11817/j.issn.1672-7347.2016.10.015.
PMID: 27807335
10
Nerve ultrasound detects abnormally small nerves in patients with spinal and bulbar muscular atrophy.
Pelosi L, Ghosh A, Leadbetter R, et al.
Muscle & nerve 2022; (65(5)):599-602 doi:10.1002/mus.27509.
PMID: 35092036
11
Differences in F-Wave Characteristics between Spinobulbar Muscular Atrophy and Amyotrophic Lateral Sclerosis.
Fang J, Cui L, Liu M, et al.
Frontiers in aging neuroscience 2016; (8()):50 doi:10.3389/fnagi.2016.00050.
PMID: 27014057
12
Metabolic alterations in spinal and bulbar muscular atrophy.
Francini-Pesenti F, Vitturi N, Tresso S, Sorarù G
Revue neurologique 2020; (176(10)):780-787 doi:10.1016/j.neurol.2020.03.020.
PMID: 32631678
13
Computed tomography-based radiological gynecomastia in SBMA as an independent differential diagnostic biomarker: a retrospective study.
Kang M, Kim BJ, Nguyen B, Park JS
Neurological sciences : official journal of the Italian Neurological Society and of the Italian Society of Clinical Neurophysiology 2025; (46(2)):783-789 doi:10.1007/s10072-024-07820-1.
PMID: 39467933
14
Clinical manifestations and AR gene mutations in Kennedy's disease.
Liu X, Zhu M, Li X, Tang J
Functional & integrative genomics 2019; (19(3)):533-539 doi:10.1007/s10142-018-0651-7.
PMID: 30612224

This page explains the genetics and diagnostic testing for Kennedy Disease for educational purposes only. Always consult a neurologist or genetic counselor to interpret your specific test results.

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