17q21.31 deletion syndrome (also known as Koolen-de Vries syndrome) is an extremely rare genetic condition caused by loss of a region on the long arm of chromosome 17 that contains the KANSL1 gene. It can involve developmental delay, distinctive facial features, and epilepsy, but the severity varies widely from person to person, and children with this condition are often described as unusually calm and friendly. Most people who search for this diagnosis have three questions in mind: what the condition actually is, whether it is inherited, and whether NIPT can detect it.
This article starts with the KANSL1 gene and a distinctive chromosomal feature called the “H2 haplotype,” then covers symptoms, diagnosis, the relationship with NIPT, and what families can do to prepare, drawing on public health authorities and peer-reviewed literature. Because the name is easily confused with other chromosome 17 conditions, we also clarify the differences.
💡 What this article covers
- The cause of 17q21.31 deletion syndrome (Koolen-de Vries syndrome) and the role of the KANSL1 gene
- Main symptoms and how they tend to change with age
- How it differs from other chromosome 17 disorders with similar-sounding names
- Whether standard NIPT can detect it, and what testing is needed for a definitive diagnosis
- Approaches to treatment and support for families
What is 17q21.31 Deletion Syndrome (Koolen-de Vries Syndrome)?
It is a congenital condition caused by loss of the 17q21.31 region on the long arm of chromosome 17, and internationally the name is increasingly standardized to “Koolen-de Vries syndrome.” The name comes from three research groups led by Dr. Koolen, Dr. Sharp, and Dr. de Vries, who independently reported the condition in 2006.
Estimates of how common it is vary by study, ranging from roughly 1 in 16,000 to about 1 in 55,000 births. Because the condition was often missed before chromosomal microarray analysis (CMA) became widespread, its true frequency is still being clarified. The U.S. rare disease information center GARD and the European rare disease database Orphanet both publish overviews of the symptoms, causes, and diagnosis.
Cause: The KANSL1 Gene and the H2 Haplotype
The condition is caused either by deletion of the KANSL1 gene within the 17q21.31 region, or by a variant within the KANSL1 gene itself. KANSL1 is involved in histone modification and is thought to influence many cellular functions, including brain development.
A second key feature is the “H2 haplotype,” a common chromosomal inversion in this region. People who carry the H2 haplotype are more likely to experience a deletion occurring during the formation of sperm or egg cells. In populations of European ancestry, roughly 20% of people carry the H2 haplotype, and the great majority of them are unaffected carriers with no symptoms.
| Mechanism | Characteristics | Inheritance pattern |
|---|---|---|
| Microdeletion of the 17q21.31 region | Multiple genes, including KANSL1, are lost together | Almost always de novo (not inherited from either parent) |
| Variant in the KANSL1 gene alone | Caused by a sequence change rather than a deletion | Autosomal dominant (a change in one copy is enough to cause symptoms) |
In both cases, most occurrences are de novo, meaning they arise from a spontaneous change during the formation of reproductive cells. Neither parent is responsible for causing it, and there is nothing a parent could have done differently. In rare cases, the change is inherited from a parent, so once a diagnosis is confirmed, a family evaluation by a clinical geneticist is recommended.
Main Symptoms and How They Change With Age
The core features are low muscle tone, developmental delay, and distinctive facial characteristics. The severity and combination of symptoms differ from child to child, and the presentation often changes as the child grows.
In infancy, low muscle tone (hypotonia) is usually the first thing families notice: the baby’s body feels unusually soft, and head control and walking tend to be delayed, along with delayed speech development. Intellectual disability is most often mild to moderate, and one of the most notable features of this condition is that affected children are frequently described as unusually friendly and easygoing.
| Symptom | Approximate frequency | Notes |
|---|---|---|
| Hypotonia / developmental delay | Nearly all cases | Often accompanied by mild-to-moderate intellectual disability |
| Distinctive facial features | Most cases | Long face, broad forehead, pear-shaped nasal tip, protruding ears |
| Epilepsy | Roughly half of cases | Onset age and severity vary widely |
| Congenital heart defects | About 30-50% of cases | Early evaluation with echocardiography is recommended |
| Genitourinary abnormalities | A subset of cases | Includes undescended testes and kidney structural differences |
These figures are approximate, and the way symptoms present varies considerably in practice. Even with the same underlying genetic change, some individuals have very mild symptoms while others require more extensive medical care. It is important not to assume a child’s future based on severity at the time of diagnosis alone.
Avoiding Confusion With Other Chromosome 17 Disorders
17q21.31 deletion syndrome affects the long arm of chromosome 17, which makes it genetically distinct from Smith-Magenis syndrome and Miller-Dieker syndrome, both of which involve the short arm. Because search results for these conditions often overlap, it helps to start with where each change occurs on the chromosome.
| Condition | Chromosomal location | Main features |
|---|---|---|
| 17q21.31 deletion syndrome (Koolen-de Vries syndrome) | Chromosome 17, long arm, 21.31 | Friendly temperament, developmental delay, distinctive facial features |
| Smith-Magenis syndrome | Chromosome 17, short arm, 11.2 | Sleep disturbance, behaviors including self-injury |
| Miller-Dieker syndrome | Chromosome 17, short arm, 13.3 | Lissencephaly (brain formation abnormality) with severe neurological symptoms |
| Williams syndrome | Chromosome 7, long arm, 11.23 | Heart defects, distinctively cheerful personality, weaker visuospatial skills |
The description “unusually friendly personality” is also commonly used for Williams syndrome, which is one reason information about the two conditions gets mixed up online. These are genetically distinct disorders, so it’s essential to rely on a confirmed diagnosis rather than a name alone.
Relationship With NIPT (Non-Invasive Prenatal Testing)
Standard NIPT, which screens for trisomy 21, 18, and 13, cannot detect 17q21.31 deletion syndrome. This is a point many expectant mothers misunderstand.
Standard NIPT analyzes fragments of fetal-derived DNA in the mother’s blood to detect changes in the “number” of chromosomes. 17q21.31 deletion syndrome involves the loss of part of a chromosome rather than a change in chromosome number, so it falls outside the scope of standard testing. Extended NIPT panels that include microdeletions and duplications do exist, but the regions each panel covers vary by testing company and plan, and not all extended panels include the 17q21.31 region. Always confirm the covered regions in the test description or during counseling before testing.
It is also worth remembering that NIPT, whether standard or extended, is a non-invasive screening test rather than a diagnostic one. Even a positive result requires confirmation through chromosomal microarray analysis (CMA) performed on a sample obtained by amniocentesis. The NIPT guidelines from the Japan Society of Obstetrics and Gynecology (Japanese) state clearly that NIPT is not a diagnostic test. A helpful overview of microarray testing is also available from the Genomic Medicine Department at Tokyo Women’s Medical University (Japanese).
If an extended NIPT panel returns a positive result for a microdeletion in the 17q21.31 region, the typical next steps are as follows.
- Genetic counseling: A specialist first explains what the result means, including the possibility of a false positive.
- Amniocentesis (CMA): Chromosomal microarray analysis of fetal cells obtained by amniocentesis confirms the diagnosis.
- Referral to specialists: If confirmed, pediatric and clinical genetics specialists help plan ongoing support.
For a broader view of the conditions NIPT can detect, see our article on conditions that can be identified through NIPT.
How the Diagnosis Is Confirmed, Before and After Birth
A definitive diagnosis is made using chromosomal microarray analysis (CMA) on fetal cells obtained through amniocentesis or chorionic villus sampling, or on a comparable genetic test performed after birth. Conventional karyotyping (G-banding) alone can miss a deletion this small, so it is not sufficient on its own.
When developmental delay or distinctive facial features raise suspicion after birth, the same microarray analysis, or a gene panel/exome test targeting KANSL1, can confirm the diagnosis. GeneReviews, a widely used international clinical genetics resource, provides detailed recommendations for testing procedures and follow-up care that physicians and clinical geneticists commonly reference when planning care.
Treatment, Developmental Support, and Outlook
There is no cure that reverses the underlying genetic change, but symptom-specific medical care combined with early developmental support can substantially improve quality of life. A diagnosis is a starting point, not an endpoint, for helping a child reach their potential.
Support typically involves several specialists working together, tailored to each child’s symptoms. Managing epilepsy and regularly evaluating heart defects are especially important in infancy and early childhood.
- Developmental support: Speech therapy and physical therapy are combined according to each child’s developmental stage.
- Epilepsy management: Anti-epileptic medication is adjusted to the seizure type, with regular EEG monitoring.
- Cardiac and genitourinary evaluation: Early echocardiography and renal ultrasound check for associated conditions.
The friendly, easygoing temperament often seen in children with this condition tends to make it easier for them to accept support, and families often report that developmental therapy shows visible results. There is, of course, individual variation, but I have seen in clinic how children who receive appropriate support early on steadily build their abilities. Rather than being overwhelmed by the diagnosis, working with specialists to plan the next step is what genuinely helps a child’s future.
Support for Families
Because long-term therapy and hospital visits are often needed, families frequently face financial and emotional strain. Connecting with public support programs and specialized institutions, rather than managing alone, makes a real difference.
Local child development support centers accept consultations even before a formal diagnosis is confirmed, from the stage when a family first notices something may be different. Support programs for children with disabilities are outlined by Japan’s Children and Families Agency; reaching out to a local support office early is worthwhile, and families should not feel they need to make these decisions alone.
What Hiro Clinic NIPT Offers
Hiro Clinic NIPT is a specialized prenatal testing clinic that supports patients from testing through result counseling and, if needed, guidance following a positive result. For rare conditions such as 17q21.31 deletion syndrome, we aim to provide explanations grounded in evidence.
We have performed over 75,000 tests to date, with a sensitivity above 99.9% and a result-reporting rate of 99.98% across all test items. Testing is processed in partnership with a domestic testing institution (Tokyo Health Inspection Center) in Japan, with results typically available in as few as 2 to 5 days. Obstetrics specialists, pediatric specialists, and clinical genetics specialists work together, and genetic counseling is provided by physicians.
If an extended panel returns a positive result for a microdeletion, our support does not end there. We offer a subsidy program to help cover the cost of confirmatory testing, such as amniocentesis, and this subsidy also applies to amniocentesis performed at another clinic. There is no age restriction and no referral letter is required. If you are unsure which test is right for you, please reach out before deciding on your own.
Related Conditions to Explore
Many other conditions, like 17q21.31 deletion syndrome, are caused by microdeletions or microduplications. If you’d like to learn more about conditions with a similar underlying mechanism, the following may help.
- 17q23.1-q23.2 microdeletion syndrome: another microdeletion on the same long arm of chromosome 17, located further along the chromosome.
- 11q22.2-q22.3 microdeletion syndrome: a distinct autosomal microdeletion that also involves developmental delay.
Frequently Asked Questions
Here are answers to common questions about 17q21.31 deletion syndrome (Koolen-de Vries syndrome). Feel free to jump to whichever question is most relevant to you.
Are 17q21.31 deletion syndrome and Koolen-de Vries syndrome the same condition?
Yes, they are the same condition. It was originally called “17q21.31 microdeletion syndrome,” but once it became clear that the same symptoms can occur from a KANSL1 gene variant alone, without any deletion, the name “Koolen-de Vries syndrome” became the increasingly preferred term.
Is it inherited from a parent, or does it happen spontaneously?
Most cases occur de novo, meaning spontaneously, and neither parent is responsible. However, when the cause is a variant in the KANSL1 gene itself, inheritance can follow an autosomal dominant pattern, and in rare cases the change has been inherited from a parent. Consulting a clinical genetics specialist after diagnosis is recommended.
Can standard NIPT detect 17q21.31 deletion syndrome?
No. Standard NIPT, which screens for trisomy 21, 18, and 13, cannot detect it. Extended panels that include microdeletions vary by testing company and plan in what they cover, and a confirmed diagnosis requires chromosomal microarray analysis (CMA) via amniocentesis.
What symptoms are most common?
The core features are low muscle tone in infancy, developmental delay, and distinctive facial characteristics. Epilepsy and congenital heart defects can also occur. Severity varies considerably from child to child, and affected children are often described as calm and unusually friendly.
How is it different from Smith-Magenis syndrome and other chromosome 17 conditions?
Even though all involve chromosome 17, 17q21.31 deletion syndrome affects the long arm, while Smith-Magenis syndrome and Miller-Dieker syndrome affect the short arm. The genes involved and the resulting symptoms are different in each case, so because the names sound similar, it’s important to confirm which diagnosis actually applies.
Is there a treatment, and what is the long-term outlook?
There is no cure for the underlying genetic cause, but developmental therapy, speech therapy, and medication for epilepsy when present can meaningfully improve quality of life. With early, appropriate support, many children make steady developmental progress.
Summary
17q21.31 deletion syndrome (Koolen-de Vries syndrome) is a rare genetic condition caused by a deletion or variant involving the KANSL1 gene. Most cases occur spontaneously, and neither parent is responsible. Standard NIPT cannot detect it, and a definitive diagnosis requires chromosomal microarray analysis via amniocentesis.
The main symptoms are developmental delay and distinctive facial features, but their presentation varies considerably from child to child, and the friendly temperament often seen with this condition can make it easier for children to accept support. Rather than letting a diagnosis define a child’s future, working with specialists on the next step is what truly helps families move forward. Please don’t hesitate to reach out if you have questions.
Medically reviewed by: Hiroshi Oka, MD, PhD — Director-General and Laboratory Director, Hiro Clinic (Fukubikai Medical Corporation). Graduate of Keio University School of Medicine, licensed to practice medicine in both Japan and the United States, and one of the few physicians in Japan to hold Laboratory Director credentials. Author of “The First Book to Read When You Learn You’re Pregnant” (Japanese title: 妊娠したら最初に読んで欲しい本). This article references public and academic sources including GeneReviews, GARD, Orphanet, the Genomic Medicine Department at Tokyo Women’s Medical University, and the Japan Society of Obstetrics and Gynecology. Reported frequencies and symptom rates vary across the literature; please consult your physician for diagnosis and treatment decisions.
日本皮膚科学会 皮膚科専門医／日本医師会 産業医／東京衛生検査所 指導監督医
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