A Guide to Genetic Testing
What is genetic testing and why is it important?
Genetic testing is the main way to confirm a diagnosis of Myofibrillar Myopathy type 13 (MFM13). This testing looks for changes (called variants or mutations) in the HSPB8 gene, which are known to cause the condition.
Because MFM13 can look similar to other muscle diseases, DNA testing is the only way to make a definite diagnosis. Getting an accurate result helps your doctors:
- Understand whether other tests or screenings are needed (for example, for heart or breathing function),
- Make personalized care recommendations,
- Identify whether other family members might carry the same change,
- And provide opportunities to join research studies to help advance understanding of MFM13.
How do I get the DNA test for MFM13?
If you are interested in genetic testing for MFM13, please talk to your doctor, such as a neurologist, or genetic counselor. A genetic counselor specializes in inherited conditions and can help you understand your testing options and what the results might mean for you and your family.
Testing is usually done with a saliva sample or a small blood sample (less than one tablespoon). Results are typically available in about three to five weeks.
There are a few different ways that your doctor can order the HSPB8 test:
- Some tests look only at specific sections of the HSPB8 gene where mutations are known to occur.
- Others test the entire HSPB8 gene or include it as part of a larger panel of genes related to muscle and nerve disorders.
If another family member has already been diagnosed with an HSPB8 variant, your doctor can order a single-gene test that looks specifically for that same change.
If it’s not yet clear what is causing the symptoms, your doctor may order a multi-gene panel, which looks at many genes at once.
In many cases, HSPB8 can also be added (by request) to existing diagnostic panels such as:
- Congenital myopathy panels
- Comprehensive myopathy panels
- Distal myopathy panels
- Comprehensive neuromuscular disorder panels
Whole Exome and Whole Genome Sequencing
Sometimes, broader testing such as Whole Exome Sequencing (WES) or Whole Genome Sequencing (WGS) is used. These tests look at nearly all genes in the body and can sometimes find rare variants that smaller panels might miss.
Not every test will find the exact cause, but in our experience, GeneDx and Ambry Genetics have successfully identified HSPB8 mutations causing MFM13.
Your doctor and a genetic counselor can help you choose the best testing option and interpret the results. Doctors ordering these tests usually include detailed clinical notes and documentation to help with insurance approval.
Where can I get tested?
Your doctor or genetic counselor can help you choose a testing laboratory.
A list of laboratories that include HSPB8 in their genetic testing panels can be found at the bottom of this page.
How does DNA testing work?
DNA from your saliva or blood sample is compared to a reference (a “normal” or “healthy” version of the gene). If a difference (variant) is found, specialists determine whether that change could cause MFM13.
There are two main reasons for testing:
- To confirm a diagnosis in someone who already has symptoms, and
- To check for a familial variant in someone with a family history of MFM13, even if they don’t have symptoms yet.
If possible, it’s best to test a family member who has symptoms first.
Once the mutation running in the family is known, testing other relatives becomes easier, less expensive, and more accurate.
Understanding your genetic test results
Genetic test results can sound complicated, but here’s what they usually mean:
- Pathogenic variant → a known disease-causing change in the HSPB8 gene. This confirms a diagnosis of MFM13.
- Likely pathogenic variant → very likely to cause disease, but more evidence is needed to be certain.
- Variant of Unknown Significance (VUS) → a change that hasn’t been seen before, and we don’t yet know if it causes disease.
If a VUS is found, doctors may recommend additional testing or checking whether other family members have the same change. Sometimes, as more information becomes available, these results are reclassified. Once a diagnosis is confirmed, testing other family members (carrier testing) is often recommended.
What happens if my test is positive?
A positive result means that a mutation causing MFM13 was found in your HSPB8 gene.
If you already have symptoms, this confirms the diagnosis.
If you don’t yet have symptoms, it means you are likely to develop the disease in the future or could pass it on to your children.
What happens if my test is negative?
A negative result means no mutation was found in the parts of the HSPB8 gene that were tested.
If your family has a known HSPB8 mutation and you test negative, this means you will not develop MFM13.
What does a genetic test report look like?
An example of a genetic test report (from GeneDx) is below
What are the risks or things to consider?
If you do not have symptoms but are considering testing, it’s important to think about what the results might mean for you and your family. Some people want to know; others prefer not to.
A genetic counselor can help you think through these questions and make an informed decision.
Concerns about discrimination based on genetic results have existed in the past, but the Genetic Information Nondiscrimination Act (GINA) passed in 2008 protects people in the U.S. from discrimination by health insurers and employers based on genetic information.
