What Do CAG Repeat Numbers Mean in Huntington's Disease?

Receiving genetic test results for Huntington’s disease (HD) can be an incredibly overwhelming and emotional experience. The most important piece of information on your report is your CAG repeat number. This number measures how many times a specific sequence of DNA (C-A-G) repeats within your HTT gene. Your CAG number determines whether you will develop the disease and gives a rough estimate of when symptoms might begin ^[1]. However, it cannot predict the exact day or year you will start noticing changes.

The Four Categories of CAG Repeats

Clinical guidelines divide CAG repeat lengths into four specific categories. Because the HTT gene can be somewhat unstable when passed from parent to child, these categories explain both your own health and the potential risks for your future children:

• Normal (26 or fewer repeats): You do not have the genetic mutation for HD and will not develop the disease. You will not pass the disease on to your children.
• Intermediate (27 to 35 repeats): You will not develop HD symptoms yourself ^[2]. However, the repeat number can increase (expand) when passed to your children, particularly if inherited from a father ^[3][2]. This means you are passing on an unstable gene that could expand into the disease-causing range in the next generation.
• Reduced Penetrance (36 to 39 repeats - May or May Not Develop Symptoms): In this range, you may or may not develop symptoms of HD during your lifetime ^[2]. If symptoms do appear, they often begin later in life and may be milder than average ^[4]. Each of your children has a 50% chance of inheriting this expanded gene, and the repeat number can expand further into the “full penetrance” range when passed down ^[2].
• Full Penetrance (40 or more repeats - Will Develop Symptoms): A number of 40 or higher means that you will eventually develop symptoms of HD if you live a typical lifespan ^[1]. Just like the reduced penetrance category, you have a 50% chance of passing the expanded HD gene to each of your children.

How the Number Affects When Symptoms Start

For those in the reduced or full penetrance ranges (36 repeats and above), there is a strong link between the CAG repeat number and when symptoms first appear:

• Higher numbers generally mean earlier onset: People with higher CAG counts tend to develop symptoms earlier in life and experience faster disease progression ^[1][5]. For example, numbers above 60 are often associated with juvenile-onset HD ^[1].
• The number is not a perfect clock: While the CAG count is the strongest predictor of when symptoms will start, it is not the only factor ^[1]. People with the exact same CAG number can start experiencing symptoms at very different ages.
• Other genes play a role: Recent research has found that other genes—especially those involved in repairing DNA—influence how quickly the disease progresses ^[6]. A process called somatic expansion, where the CAG repeat number actually increases over time in certain brain cells as we age, is a key driver of when symptoms begin ^[7][6].

Proactive Steps and Family Planning

Learning that you or your children are at risk can cause significant anxiety, but you have options. If you are in the intermediate, reduced, or full penetrance ranges, it is strongly recommended that you work with a certified genetic counselor. A genetic counselor can help you interpret your exact numbers and discuss family planning options. For instance, processes like in vitro fertilization (IVF) combined with preimplantation genetic testing can allow you to have children without passing on the HD gene.

You do not have to navigate this alone. Building a care team that includes a neurologist who specializes in movement disorders, a genetic counselor, and a mental health professional is a critical next step. Organizations like the Huntington’s Disease Society of America (HDSA) can also connect you with support groups and resources to help you process this life-changing information.