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Genetic Testing for Bloom Syndrome
While Bloom syndrome is rare, people of Ashkenazi Jewish descent have much higher odds than the general population of carrying the genetic mutation responsible for the disease. Since carriers have no symptoms, most families don't find out they're at risk until they have a child with the condition. A carrier screen for Bloom syndrome checks for the genetic mutations associated with the disease so you can understand your risk before you begin planning a pregnancy.
What Is Bloom Syndrome?
Bloom syndrome is caused by a change in a gene responsible for repairing damaged DNA. Without a working copy of that gene, cells can't properly fix the small errors that build up over time, and this leads to widespread instability across a person's chromosomes.1,2
Children born with Bloom syndrome are usually much smaller than average, both before and after birth, and they often have a narrow face, with a nose and ears that appear more prominent than expected. Many children with Bloom syndrome develop a reddish butterfly-shaped rash across the nose and cheeks that worsens with sun exposure. They can also suffer from a weakened immune system and frequent infections, and they have a higher chance of diabetes.1 The condition also affects fertility. Most men with Bloom syndrome cannot father children, and many women reach menopause earlier than expected.1
The most serious health risk for people with Bloom Syndrome is an increased risk of cancer. Affected individuals face a significantly higher chance of developing cancer, often starting in childhood or the teenage years. By age 40, more than 80% of people with Bloom Syndrome will have been diagnosed with at least one type of cancer, most often leukemia, lymphoma, or colorectal cancer. Overall, their cancer risk is estimated to be 150 to 300 times that of the general population.1 There is no cure for Bloom syndrome. Care focuses on strict sun protection, cancer monitoring that starts in childhood, and treatment plans adjusted for the body's heightened sensitivity to radiation and certain chemotherapy drugs.1,2
BLM
Bloom syndrome is inherited in an autosomal recessive pattern. The gene associated with Bloom Syndrome is called BLM. Like most genes, everyone has two copies of BLM, one inherited from each parent. When both copies carry a disease-causing change, the body can’t repair DNA correctly, which leads to the health complications described above.1,2
A person with one working copy of BLM and one nonworking copy is called a carrier. Carriers don’t have any symptoms, because one working copy is enough to repair DNA. Bloom Syndrome only develops when a person inherits a nonworking copy from both parents
If both parents are carriers, each pregnancy has:2
- A 25% chance the child inherits a nonworking copy from each parent and is affected by Bloom Syndrome
- A 50% chance the child is an unaffected carrier, like the parents
- A 25% chance the child inherits two working copies and is unaffected
Why Carrier Screening for Bloom Syndrome Matters
There’s no cure for Bloom syndrome, and a child born with the condition will face an increased cancer risk throughout life. Carrier screening is the one tool that can tell you your risk before pregnancy even begins. Because carriers have no signs of the condition, screening is the only reliable way to find out whether you carry a nonworking copy of BLM.
That’s why Bloom syndrome is included on most expanded carrier screening panels. The American College of Medical Genetics and Genomics (ACMG) recommends offering carrier screening for Bloom syndrome to everyone who is pregnant or planning a pregnancy, not only people with a known family history.2
Your Odds of Carrying the BLM Gene
Bloom syndrome is very rare, with about 300 cases reported worldwide. Even so, carrier rates are meaningful in some communities. In the United States, about 1 in 157 people of Ashkenazi Jewish descent carry a change in BLM, and in Israel the rate is higher, at roughly 1 in 111.1,2
That doesn’t mean Bloom syndrome only affects Ashkenazi Jewish families. About three out of every four reported cases occur in people without Jewish ancestry, so anyone can be a carrier, whatever their background.1
Who Should Consider Screening
Ashkenazi Jewish Ancestry
The carrier rate in this population is high enough that ancestry alone is often reason to get screened, regardless of family history.
Planning a Pregnancy
If you're planning a pregnancy, screening ahead of time gives you the most information and the fullest range of options before you conceive.
A Partner Who Has Already Screened Positive
If your partner has already screened positive, you'll need to know your own carrier status to understand whether your child could be at risk.
Using a Sperm or Egg Donor
Donor gametes carry the same genetic considerations as a partner would, so screening applies here just as it does with a partner.
Bloom Syndrome in Your Immediate Family
If Bloom syndrome runs in your immediate family, your own chance of carrying the gene change is higher than average, which makes screening worth a closer look.
How At-Home Genetic Testing Works
Order and register
Collect your sample
Return your sample
Get your results
Talk to a genetic counselor
Costs and Insurance Coverage
Insurance Billing
Self-Pay Option
Understanding Your Results
Positive (Carrier)
A positive result means you carry one working copy and one nonworking copy of BLM.1,2 You don’t have Bloom syndrome and won’t develop it, but you can pass the nonworking copy on to a child. If you’re planning a pregnancy, your partner should be screened next. Your child’s risk depends on whether your partner also carries a change in BLM.
Negative Result
A negative result means your chance of being a carrier for Bloom Syndrome is much lower than before. It doesn’t drop to zero, though, because no screening test can detect every possible change in BLM.
If You and Your Partner Are Both Carriers
If both partners carry a change in BLM, each pregnancy has a 25% chance of being affected by Bloom syndrome.2 If you are already pregnant, diagnostic tests such as chorionic villus sampling or amniocentesis can determine whether the current pregnancy is affected, giving you an answer before birth.3 Genetic counseling is strongly recommended for carrier couples, and every jscreen result comes with access to a board-certified genetic counselor who can walk you through what your results mean.
Know Your Risk Before You're Expecting
Frequently Asked Questions
What is Bloom syndrome?
Bloom Syndrome is a rare hereditary disorder caused by changes in both copies of the BLM gene. This gene normally makes a protein that helps repair damaged DNA. Without a working copy, genetic damage builds up quickly, which greatly increases the risk of cancer.
What symptoms does a person with Bloom syndrome experience?
People with Bloom syndrome may have:
- Small size at birth and short stature throughout life
- A red facial rash that gets worse with sun exposure
- A weakened immune system, leading to frequent infections
- Infertility in men and reduced fertility in women
- A very high risk of many types of cancer, often at young ages
People who have a change in only one copy of BLM (carriers) don’t have Bloom syndrome. Some research suggests they may have a modestly higher risk of colorectal and possibly other cancers, and studies in this area are ongoing.
How common is Bloom syndrome?
Bloom syndrome is very rare, with about 300 cases reported worldwide. It’s more common in people of Ashkenazi Jewish ancestry, about 1 in 157 of whom carry a change in the BLM gene.
How is Bloom syndrome treated?
There is no cure for Bloom Syndrome. Sun protection is critical to reduce skin damage. Cancer screening is essential and should cover multiple cancer types. Immune deficiency may require preventive antibiotics or antibody replacement therapy.
What is the outlook for individuals with Bloom syndrome?
Life expectancy for those with Bloom Syndrome is significantly reduced due to the very high cancer risk they face. Many people with Bloom Syndrome develop cancer in their 20s and 30s, but intensive cancer surveillance can improve outcomes through early detection.
What Does a Bloom Syndrome Genetic Test Show?
Results generally fall into two categories. A negative result means you're very unlikely to be a carrier, though it can't rule out every possible genetic mutation. A positive result means you carry one working copy and one nonworking copy of BLM, with no health effects to you personally. If you and your partner are both carriers, you are at risk of having a child with Bloom Syndrome.
Is There a Diagnostic Test for Bloom Syndrome While Pregnant?
If you're already pregnant and both partners are found to be carriers, diagnostic testing such as chorionic villus sampling or amniocentesis can determine whether the pregnancy is affected, rather than waiting until birth.3
How Do You Test for Bloom Syndrome From Home?
With jscreen, a carrier screening happens entirely from home. You order a kit online, provide a saliva sample using the instructions included, and mail the test kit back using the prepaid materials in the kit. Your sample is analyzed at a CLIA-accredited laboratory, and results are typically available within 3 to 4 weeks.
Why Is Bloom Syndrome More Common Among Ashkenazi Jewish People?
A specific change in the BLM gene became more common in the Ashkenazi Jewish population generations ago. Certain genetic diseases are more common within communities that were historically isolated – whether geographically, culturally, or religiously. When one or more ancestors in such a group carried a mutation in a disease-related gene, that mutation had the opportunity to spread and take root over generations. These types of genetic mutations are known as "founder mutations,” which is why about 1 in 157 people with Ashkenazi Jewish ancestry carry it today, compared to a much lower rate in the general population.1
Do Bloom Syndrome Carriers Have a Higher Cancer Risk?
Carrying one non-working copy of BLM doesn’t cause Bloom Syndrome. A few studies have looked at whether carriers may have a slightly higher risk of certain cancers, such as colorectal cancer, but the evidence is limited and mixed. Current medical guidelines don’t recommend extra cancer screening for carriers unless they have other risk factors, such as a family history of cancer.²
Do I Need Genetic Counseling After My Results?
We highly encourage everyone to schedule a genetic counseling session, regardless of your test results. Screening through jscreen includes a no-cost consultation with a board-certified genetic counselor who can explain your specific results and your options are going forward.
What If My Partner and I Are Both Bloom Syndrome Carriers?
When both parents are carriers, each pregnancy has:
- A 25% chance of the child being affected with Bloom syndrome
- A 50% chance of the child being an unaffected carrier
- A 25% chance of the child inheriting two working copies¹
Working with a genetic counselor will help a carrier couple talk through their options, which can include prenatal diagnostic testing during a current pregnancy, in vitro fertilization with preimplantation genetic testing before conceiving, or other family planning approaches.
References
- Bloom Syndrome. National Organization for Rare Disorders (NORD). Last updated February 27, 2024. Accessed September 9, 2026.
https://rarediseases.org/rare-diseases/bloom-syndrome/ - Flanagan M, Cunniff CM. Bloom Syndrome. GeneReviews. University of Washington, Seattle. Published March 22, 2006. Updated October 12, 2023. Accessed September 9, 2026. https://www.ncbi.nlm.nih.gov/books/NBK1398/
- Chorionic Villus Sampling (CVS). Cleveland Clinic. Last reviewed August 8, 2024. Accessed September 9, 2026. https://my.clevelandclinic.org/health/diagnostics/4028-chorionic-villus-sampling-for-prenatal-diagnosis