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Hereditary Multiple Osteochondromas

Also called: Hereditary multiple extoses, multiple cartilaginous extoses, diaphyseal aclasis

Hereditary multiple osteochondromas (HMO) is a rare genetic condition. People with hereditary multiple osteochondromas are more likely to develop certain types of tumors. These tumors can be benign (non-cancerous) or malignant (cancerous).

Children with this condition can develop 1 or more tumors in their bones. These tumors are called osteochondromas. They are typically located near the metaphyses (growth plates or ends) of the bone.

Cancer risk with hereditary multiple osteochondromas

Osteochondromas are benign, but they may become malignant over time. People with HMO have a 95% chance of developing non-cancerous osteochondromas by age 12.

The malignant form of osteochondroma is called osteochondrosarcoma. People with HMO have a 2–5% lifetime risk of developing osteochondrosarcoma.

Other health problems with hereditary multiple osteochondromas

People with HMO may have other symptoms that result from the development of osteochondromas. These symptoms may vary from person to person, even within the same family. These symptoms include:

  • Reduced skeletal growth
  • Restricted joint motion
  • Short stature
  • Premature osteoarthritis
  • Nerve compression
  • Bowing of forearm or ankle
  • Abnormal development of hip joints
  • Difficulty walking and/or discomfort when walking

Causes of hereditary multiple osteochondromas

HMO is caused by changes in 1 of 2 genes: EXT1 or EXT2. Genes carry information telling cells within the body how to function. The role of EXT1 and EXT2 in tumor development is not understood. It is thought the normal functions of EXT1 and EXT2 help prevent tumors.

Most cells in the body carry 2 working copies of the EXT1 and EXT2 genes. One copy of each gene is passed down from each parent. People with HMO carry 1 working copy of the EXT1 or EXT2 gene and 1 non-working copy. This non-working copy is also called a mutation. When the remaining copy of the EXT1 or EXT2 gene becomes damaged, it can lead to osteochondromas.

How hereditary multiple osteochondromas is inherited

Most people with HMO inherit it from a parent who also has the condition.

Some HMO cases happen due to a new EXT1 or EXT2 mutation that was not passed down from a parent. In these cases, the change happened in either the egg or the sperm cell when the child was formed. Or the change occurred in 1 of the child’s cells early in development. These children are the first in their families to have HMO.

People with HMO have a 50% (1 in 2) chance of passing on the EXT1 or EXT2 mutation to each of their children.

Hereditary multiple osteochondromas diagnosis

Health history

A health care provider may suspect that your child has HMO after studying their medical and family cancer history.

This information helps the health care provider and the genetic counselor know if:

  • There are more tumors than normal in your family
  • The tumors happened at a younger age than expected
  • The tumors are those that are seen in HMO

Your provider or genetics counselor may recommend EXT1 and EXT2 genetic testing if they suspect HMO. Sometimes EXT1 and EXT2 are included in genetic testing, even if HMO is not suspected.

Learn more about types of genetic tests.

Genetic testing for hereditary multiple osteochondromas

A blood sample is sent to a genetic testing lab. The lab runs genetic tests that look for changes in the EXT1 or EXT2 gene.

If your child has an EXT1 or EXT2 mutation, the genetic counselor will work with your family to:

  • Find out if other family members should consider testing for an EXT1 or EXT2 mutation
  • Help your family understand the increased cancer risks with HMO
  • Give you information to help with your decisions about prenatal genetic testing

Genetic testing before or during pregnancy

Parents may choose to do prenatal testing to find out if a pregnancy is affected with a known EXT1 or EXT2 mutation. Testing may take place either before pregnancy occurs or during pregnancy.

You should work with a genetic counselor to review the pros and cons of the test. The genetic counselor can also help you prepare for the test results.

Testing before pregnancy is called preimplantation genetic testing (PGT). This special type of genetic testing is done along with in vitro fertilization (IVF). PGT tests embryos for a known EXT1 or EXT2 mutation before a provider places the embryo into the uterus.

Testing during pregnancy can help providers see if a pregnancy has a known EXT1 or EXT2 mutation. A doctor gathers cells from the pregnancy in 1 of 2 ways:

  • Chorionic villus sampling (CVS) is done during the first 3 months of pregnancy (1st trimester).
  • Amniocentesis is done after the first 3 months of pregnancy (2nd trimester or later).

After tissue collection, the lab checks the sample for the EXT1 or EXT2 mutation. Both tests carry minor risks. Discuss risks with an experienced health care provider or a genetic counselor.

Special concerns for genetic testing

Take time to think carefully about the benefits and risks of genetic testing. Speak with a genetic counselor before testing. If you decide to get tested, talk with your health care provider or a genetic counselor about your results so you can understand what they mean.

Sometimes, children or adults with HMO feel sad, anxious, or angry after getting their test results. Parents may feel guilty if they pass the EXT1 or EXT2 gene mutation to 1 or more of their children. People with an EXT1 or EXT2 gene mutation may also have trouble getting disability or life insurance.

Read more about genetic discrimination.

Monitoring and follow-up care for hereditary multiple osteochondromas

In general, tumor screening involves undergoing certain tests to check for tumors before symptoms occur. The goal is to detect tumors at the earliest and most treatable stage. Tumor screening tests should be discussed with a health care provider who knows this condition well.

Screening tests may include the following:

  • Yearly physical exams by a health care provider who is familiar with the condition
  • MRIs, x-rays, or CT scans to check for tumors

Living with hereditary multiple osteochondromas

People with HMO should watch closely for general signs or symptoms that could signal cancer:

  • Unexplained weight loss or fever
  • Lack of appetite
  • Aches, pains, lumps, and/or swelling that cannot be explained

It is important to seek medical help if anything unusual appears.

Maintain a healthy lifestyle

Habits that will help you maintain a healthy lifestyle include:

  • Eat a healthy diet with lots of fruit and vegetables.
  • Get regular exercise.
  • Avoid excess sun exposure. Always wear a hat, protective clothing, and sunscreen (SPF 30 or higher) when out in the sun.
  • Avoid smoking or the use of tobacco products.
  • Avoid being around secondhand smoke.
  • Avoid excessive alcohol use.

Hereditary multiple osteochondromas care at
St. Jude

The St. Jude Cancer Predisposition Program is dedicated to diagnosing and screening children with a genetic predisposition for cancer. Our program offers the best clinical care possible. We engage in cutting-edge research to gain more information about genetic disorders and cancer, and to improve the care and treatment for patients who are affected by these conditions. Learn more about the Division of Cancer Predisposition at St. Jude.

Our genetic counselors, clinicians, genetics nurse practitioners, and research associates work together to provide patients with:

  • A complete health history
  • A complete family cancer history
  • Physical exams to check for features of cancer genetic conditions
  • Confidential genetic counseling and testing
  • Cancer screenings for those with genetic disease to detect and treat cancer as soon as it occurs
  • Recommended ways to reduce cancer risk
  • Genetic testing for immediate (1st degree) relatives
  • Research opportunities related to cancer history and/or genetic diagnosis

HMO increases your child’s risk for tumors.

Related clinical trials

St. Jude offers clinical trials and cancer research studies for children, teens, and young adults for hereditary multiple osteochondromas. Learn more about clinical research at St. Jude.

GENETX: Assessing Gene Therapy Communication Needs

Study goal:

To learn more about participants beliefs, attitudes, and questions about gene therapy to help make web-based resources to share information. This could help patients make treatment decisions.


18 to 35 years old with rare genetic diseases

Published results
G4K: Genomes for Kids

Study goal:

The main goal of this study is to learn more about the reasons childhood tumors form and how to treat them better.

TBANK: Collecting, Banking, and Distributing Human Tissue Samples in St. Jude Biorepository

Study goal:

To provide a high-quality repository of tumor and normal samples to facilitate translational research performed by St Jude faculty and their collaborators


All diseases

PG4KDS: Clinical Implementation of Pharmacogenetics

Study goal:

1) Test each patient for hundreds of gene variations that might be important for drug use. All of the gene test results will be in the research laboratory, but as time goes on, the study will evaluate scientific evidence and selectively move test results for a few genes into the medical record if the evidence strongly shows that the result can help in better prescribing of drugs for patients; 2) Estimate how often pharmacogenetic test results are moved from research tests into a patient’s medical record; 3) Use methods to choose which of these tests should be put in the medical record; 4) Use computer-based tools in the electronic medical record to help doctors use gene test results when prescribing drugs; 5) Share feelings and concerns of patients and their families about gene test information being put in their medical record.

SJFAMILY: Study of Cancer in Families

Study goal:

The main purpose of this trial is to learn about the genetic causes of cancer. 


Younger than 50 years old

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Seeking treatment at St. Jude

Patients accepted to St. Jude must have a disease we treat and must be referred by a physician or other qualified medical professional. We accept most patients based on their ability to enroll in an open clinical trial.

How to seek treatment

Contact the Physician / Patient Referral Office

Call: 1-888-226-4343 (toll-free) or 901-595-4055 (local)  | Fax: 901-595-4011 | Email: | 24-hour pager: 1-800-349-4334


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