CMT1A
CMT1A is the most common form of Charcot-Marie-Tooth disease. It is caused by having an extra copy of the PMP22 gene, which disrupts the insulation around your nerves and slows the signals they carry.
What CMT1A is
CMT1A is caused by having an extra copy of a gene called PMP22. Most people carry two copies of PMP22, one from each parent. In CMT1A there are three, because a small stretch of chromosome 17 that contains the gene is duplicated. That extra copy makes the body produce too much of a protein called peripheral myelin protein 22.
PMP22 is a building block of myelin, the insulating sheath that wraps around your nerves much like the plastic coating around an electrical wire. Myelin lets nerve signals travel quickly and cleanly between your spinal cord and your muscles and skin. When there is too much PMP22, the myelin does not form or hold up properly, and the signals slow down. This is why CMT1A is called a demyelinating neuropathy, and why nerve conduction testing usually shows clearly slowed signals, often below 38 meters per second.
Because your nerves are longest reaching the feet and hands, CMT1A is length-dependent: the longest nerves are affected first, so symptoms usually begin in the feet and lower legs and only later reach the hands. Over many years, some of the nerve fibers themselves (the axons) are gradually lost. Researchers believe it is this slow loss of axons, more than the demyelination alone, that drives the increase in disability people may notice in adulthood.
How common it is. CMT1A is the most common form of CMT. In large patient cohorts it accounts for about 40 to 60 percent of genetically confirmed cases, and about 70 percent of the demyelinating (CMT1) group on its own.
Common signs and symptoms
Reported symptoms and their severity vary from person to person, even within the same family. This is a general picture, not a prediction for any one person.
- Weakness and muscle wasting that begins in the feet and lower legs, with the legs affected earlier and more than the arms
- High-arched feet (pes cavus) and curled toes (hammertoes)
- Foot drop and a high-stepping (steppage) gait, often noticed first as tripping, frequent falls, or trouble running
- Reduced or absent reflexes, especially at the ankles
- Reduced sensation in the feet and lower legs, which can affect balance
- Pain, reported by a majority of people over time, most often from the mechanical strain of foot deformity and altered walking, though some also have nerve-related (neuropathic) pain
- Fatigue, which is commonly reported and can affect daily life
- Markedly slowed nerve conduction on testing, a hallmark of the demyelinating process
- Less commonly, a hand tremor, scoliosis (a curve of the spine), or mild hearing changes
Onset and how it changes over time
Symptoms of CMT1A usually appear in childhood or adolescence, often within the first two decades of life, though the age of onset varies widely and some people are not diagnosed until adulthood. Even within the same family, one person may have clear symptoms in childhood while a relative with the very same duplication has only mild or subtle findings.
CMT1A progresses slowly and is not life-threatening. Most people keep walking throughout their lives, though many use ankle-foot braces and some use other mobility aids over time, and fewer than 1 in 20 people become dependent on a wheelchair. Because the pace and severity differ so much from person to person, your own care team is the best guide to what to expect.
How it is inherited
The duplication that causes CMT1A is dominant, which means a single copy is enough to cause CMT. A parent who has CMT1A has about a 1 in 2 (50 percent) chance of passing it to each child, sons and daughters alike. In many families it is handed down from an affected parent, but in some people the duplication happens for the first time (a de novo change), so there is no family history. A genetic counselor can help you understand what this means for you and for family planning.
Autosomal dominant inheritance
One changed copy of the gene is enough to cause CMT. A parent who has it passes it to about half of their children, on average, sons and daughters alike.
- Affected
- Carrier
- Unaffected
- Male
- Female
This diagram shows a typical family. Real families vary, and it does not predict any specific outcome. A genetic counselor can walk through what it means for yours.
Faces of CMT
Real families living with CMT1A, in their own words.
Bernadette Scarduzio
HNF's longtime national spokesperson and the subject of the first full-length documentary on CMT.
Read their story ›
Allison Moore
HNF's founder and CEO, living with CMT1A, who turned a hospital mishap into a foundation built to make CMT a household name.
Read their story ›
Addie · Addie's Tale
Diagnosed young with CMT1A, Addie meets every appointment and brace as a champ, and her family wants a world without CMT.
Read their story ›
Jennifer DeSetto
Diagnosed at 41, months after losing her father to CMT complications. One video reached a national audience and she has been talking about it ever since.
Read their story ›Frequently asked questions
What is CMT1A?
CMT1A is the most common form of Charcot-Marie-Tooth disease. It is caused by having an extra copy of a gene called PMP22, so the body makes too much peripheral myelin protein 22. PMP22 is a building block of myelin, the insulating sheath around your nerves, and when there is too much of it the myelin does not form or hold up properly, which slows nerve signals. This is why CMT1A is called a demyelinating neuropathy.
What gene causes CMT1A?
CMT1A is caused by a duplication of the PMP22 gene on chromosome 17. Most people carry two copies of PMP22, one from each parent, but in CMT1A a small stretch of the chromosome is duplicated, leaving three copies. That extra copy makes the body produce too much protein, which disrupts the myelin around your nerves. A genetic test can confirm your subtype.
What are the symptoms of CMT1A?
Because your nerves are longest reaching the feet and hands, symptoms usually begin in the feet and lower legs and only later reach the hands. Common features include weakness and muscle wasting in the feet and lower legs, high-arched feet (pes cavus) and curled toes, foot drop with a high-stepping gait, reduced or absent ankle reflexes, and reduced sensation that can affect balance. Many people also report pain and fatigue over time, and nerve conduction testing typically shows markedly slowed signals.
How is CMT1A inherited, and will my children get it?
The PMP22 duplication that causes CMT1A is dominant, which means a single copy is enough to cause CMT. A parent who has CMT1A has about a 1 in 2 (50 percent) chance of passing it to each child, sons and daughters alike. In some people the duplication happens for the first time, so there is no family history. A genetic counselor can help you understand what this means for you and for family planning.
Is CMT1A progressive or life-threatening?
CMT1A progresses slowly and is not life-threatening. Most people keep walking throughout their lives, though many use ankle-foot braces and some use other mobility aids over time, and fewer than 1 in 20 people become dependent on a wheelchair. Because the pace and severity differ so much from person to person, even within the same family, your own care team is the best guide to what to expect.
Confirm your subtype
If you have a clinical CMT diagnosis but no confirmed gene, genetic testing is how you learn your subtype. CMT Genie helps you and your provider get tested and connects you with telehealth genetic counseling.
Clinical information on this page is based on peer-reviewed literature (including van Paassen et al., Orphanet Journal of Rare Diseases, 2014) surfaced via Consensus and cross-checked against GeneReviews (NBK1205) and OMIM 118220. Verified 2026-07-08.