CMT1B
CMT1B is caused by changes in the MPZ gene, which makes the single most abundant protein in the myelin around your nerves. Depending on the exact change, it can look like an early, more severe demyelinating neuropathy or a later, milder axonal one.
What CMT1B is
CMT1B is caused by changes in the MPZ gene, which makes myelin protein zero, the most abundant protein in peripheral myelin. Myelin protein zero acts like the glue and mortar that hold the many layers of myelin together around each nerve fiber.
MPZ is unusual among CMT genes because different changes in it can produce quite different diseases. Some changes cause an early-onset, more severe demyelinating neuropathy that begins in infancy or childhood. Others cause a later-onset, slowly progressive form that behaves more like an axonal (type 2) neuropathy in adulthood.
Because of this, the exact MPZ change matters a great deal for what to expect, which is one reason a precise genetic diagnosis is so valuable in CMT1B.
How common it is. CMT1B is one of the more common demyelinating forms of CMT after CMT1A, accounting for about 5 to 10 percent of genetically confirmed cases in large patient cohorts.
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 wasting starting in the feet and lower legs, with high arches and foot drop
- Reduced or absent reflexes and reduced sensation in the feet
- In early-onset forms, more significant childhood weakness and delayed walking
- In later-onset forms, a milder, slowly progressive course beginning in adulthood, sometimes with pupil changes or hearing involvement
Onset and how it changes over time
The age of onset in CMT1B depends heavily on the specific MPZ change. Early-onset forms can appear in infancy or early childhood and be more severe, while late-onset forms may not cause symptoms until the fourth decade or later and then progress slowly.
For most people CMT1B is slowly progressive and not life-limiting. Because the same gene produces such different courses, knowing your exact variant, and having it interpreted by someone experienced, is especially useful here.
How it is inherited
CMT1B is inherited in an autosomal dominant pattern, so a single changed copy of MPZ is enough to cause it, and an affected parent has about a 1 in 2 (50 percent) chance of passing it to each child. New changes also occur. Because the specific variant shapes the outlook so strongly, a genetic counselor can help you understand what your particular change is likely to mean.
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.
Frequently asked questions
What is CMT1B?
CMT1B is a form of Charcot-Marie-Tooth disease caused by changes in the MPZ gene, which makes myelin protein zero, the most abundant protein in the myelin around your nerves. Myelin protein zero acts like the glue and mortar that hold the many layers of myelin together around each nerve fiber. Depending on the exact change, CMT1B can look like an early, more severe demyelinating neuropathy or a later, milder axonal one.
What gene causes CMT1B?
CMT1B is caused by changes in the MPZ gene (myelin protein zero). MPZ is unusual among CMT genes because different changes in it can produce quite different diseases, so the exact change matters a great deal for what to expect. That is one reason a precise genetic diagnosis is so valuable in CMT1B, and a genetic test can confirm your subtype.
What are the symptoms of CMT1B?
Symptoms usually start in the feet and lower legs, with weakness and wasting, high arches, and foot drop, along with reduced or absent reflexes and reduced sensation in the feet. In early-onset forms there may be more significant childhood weakness and delayed walking. In later-onset forms the course is milder and slowly progressive, beginning in adulthood, and sometimes involves pupil changes or hearing involvement.
How is CMT1B inherited, and will my children get it?
CMT1B is inherited in an autosomal dominant pattern, so a single changed copy of MPZ is enough to cause it, and an affected parent has about a 1 in 2 (50 percent) chance of passing it to each child. New changes can also occur. Because the specific variant shapes the outlook so strongly, a genetic counselor can help you understand what your particular change is likely to mean.
Is CMT1B progressive or life-limiting?
For most people CMT1B is slowly progressive and not life-limiting. The age of onset depends heavily on the specific MPZ change: early-onset forms can appear in infancy or early childhood and be more severe, while late-onset forms may not cause symptoms until the fourth decade or later and then progress slowly. Because the same gene produces such different courses, knowing your exact variant and having it interpreted by someone experienced is especially useful here.
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 GeneReviews (Charcot-Marie-Tooth overview, NBK1358) and OMIM 118200. Verified 2026-07-08.