Xp21 Microdeletion Syndrome

Introduction

If expanded NIPT or a genetic test after birth has led to a diagnosis of “Xp21 deletion syndrome (Xp21 microdeletion syndrome),” an unfamiliar name like this can understandably leave you feeling anxious and confused.

Here is the short answer first. This condition is a contiguous gene syndrome that occurs when three neighboring genes on the short arm of the X chromosome at Xp21 — DMD, GK, and NR0B1 — are deleted together. Each gene can also cause a separate condition on its own, but what defines this syndrome is that symptoms affecting three different systems — muscle, adrenal function, and metabolism — overlap in the same person.

This article aims to explain, as clearly as possible, what this condition is, what to keep in mind in daily life, and how it relates to prenatal diagnosis and NIPT. Alongside the medical details, we’ve included practical notes for everyday life and a message for families. The information here draws on public sources including GeneReviews Japan, the Japan Intractable Disease Information Center, the Center for Pediatric Chronic Disease Information, the Japan Society of Obstetrics and Gynecology, and Japan’s Ministry of Health, Labour and Welfare. We hope this article can serve as one signpost on the road ahead.

1. Overview: What Kind of Condition Is This?

A condition caused by the loss of “neighboring genes” together

The human body is made up of roughly 37 trillion cells, and at the center of each is a set of chromosomes carrying our genetic blueprint. Xp21 deletion syndrome occurs when genes at a specific location called “p21” on the X chromosome — one of the sex chromosomes — are lost through a small deletion.

This “p21” location holds several genes that sit right next to each other and support important functions in the body. What makes this condition distinctive is that the deletion typically removes not just one gene, but several neighboring genes at once. This is known medically as a contiguous gene syndrome. One of the genes involved, DMD, is considered one of the largest genes in the human genome, and its size is thought to make the surrounding region more prone to deletions and rearrangements. The extent of the deletion varies from case to case, and academic literature has reported cases spanning several megabases (Mb). This is an extremely rare condition — worldwide, only slightly over 100 affected males have been reported, and reports of symptomatic females are very limited.

Other names for this condition

Depending on which combination of genes is deleted, this condition is also known by several other names.

• Xp21 microdeletion syndrome
• Complex glycerol kinase deficiency
• Xp21 contiguous gene deletion syndrome

How symptoms show up depends on which genes are affected, but the condition typically involves problems in three areas at once: muscle, adrenal (hormonal) function, and metabolism. Which genes get caught up depends on how far the deletion extends. The table below summarizes the main genes involved and their roles.

Gene	Related condition	Main function
DMD	Duchenne muscular dystrophy	Blueprint for dystrophin, a protein that protects muscle cells
GK	Glycerol kinase deficiency	Blueprint for the enzyme that metabolizes glycerol produced during fat breakdown
NR0B1 (DAX1)	Congenital adrenal hypoplasia / hypogonadotropic hypogonadism	Blueprint for a transcription factor involved in the development of the adrenal glands and the hypothalamus-pituitary axis
IL1RAPL1 (when the deletion is larger)	X-linked intellectual developmental disorder 21	Blueprint for a protein involved in forming connections between neurons

For a detailed look at Duchenne muscular dystrophy on its own — its symptoms and treatment — see our article on Duchenne muscular dystrophy (Japanese). Please don’t confuse this condition with the similarly named Xp21.2 microduplication syndrome, which involves a “duplication,” not a deletion, of genes in the same region.

2. Main Symptoms and the Genes Involved

The symptoms of this syndrome are determined by exactly which genes are missing. The Xp21 region contains three key genes lined up next to each other, and losing them brings on the conditions each one is individually associated with.

① Progressive muscle weakness (loss of the DMD gene)

• Related condition: Duchenne muscular dystrophy (DMD)
• What’s happening: The body can no longer produce dystrophin, a protein needed to maintain muscle cells.
• Main symptoms:
  • Starting to walk late, or falling easily.
  • Gowers’ sign: When standing up from the floor, the child uses their hands to push off their own knees and thighs, as if climbing up their own body.
  • Calves that become hard and enlarged (pseudohypertrophy).
  • Muscle strength declines with age, and a wheelchair or respiratory support may eventually become necessary.

Ongoing monitoring centers on regular blood tests for CK (creatine kinase) levels and checks of motor function.

② Adrenal insufficiency (loss of the NR0B1/DAX1 gene)

• Related condition: Congenital adrenal hypoplasia (AHC)
• What’s happening: The adrenal glands, located above the kidneys, fail to develop properly and cannot produce hormones essential for life, such as cortisol and aldosterone. Because this can be life-threatening, it requires the earliest possible response.
• Main symptoms:
  • Salt wasting: Sodium is lost through the urine, making dehydration more likely.
  • Darkening of the skin (hyperpigmentation).
  • Poor feeding, weak weight gain, and low energy or listlessness.
  • Adrenal insufficiency (adrenal crisis): Stress such as fever or infection can trigger a shock-like state.

Roughly 60% of cases are reportedly noticed within the first month of life, and about 40% by early childhood, based on these symptoms. From adolescence onward, a shortage of gonadotropin hormones can also bring delayed puberty or infertility.

③ Metabolic abnormality (loss of the GK gene)

• Related condition: Glycerol kinase deficiency
• What’s happening: The enzyme that converts glycerol — a byproduct of fat breakdown — into usable energy stops working.
• Main symptoms:
  • Glycerol levels rise in the blood and urine.
  • On its own, this can be mild, but combined with the other symptoms it can lead to low blood sugar, vomiting, or altered consciousness.

A known lab quirk called “pseudo-hypertriglyceridemia” can make triglyceride levels on a blood test look higher than they really are, so it’s reassuring to check with your physician about what the numbers mean each time blood is drawn.

④ Other symptoms

When the deletion covers a wider area, additional symptoms beyond those above can appear.

• Intellectual developmental disorder (intellectual disability): Genes involved in brain function, such as IL1RAPL1, or Duchenne muscular dystrophy itself, can be associated with delayed language development or learning difficulties.
• Ornithine transcarbamylase (OTC) deficiency: In rare cases where the deletion extends far enough to reach the gene responsible for breaking down ammonia, hyperammonemia can occur as well.

肢帯型筋ジストロフィー2B型（したいたいがた きんジストロフィー にービーがた）【DYSF】

肢帯型筋ジストロフィー2B型（LGMD2B）の原因・症状・検査・治療の流れをやさしく解説。不安な方への情報サポート｜ヒロクリニック...

3. Cause and Inheritance

Why does this happen?

In most cases, a portion of a chromosome is lost by chance (a spontaneous mutation) — either while eggs or sperm are being formed, or during the very earliest cell divisions of the fertilized egg. It is no one’s fault, and simply a natural event that happens by chance.

Inheritance pattern: X-linked recessive

Because this condition involves an abnormality on the X chromosome, one of the sex chromosomes, how it shows up differs by sex. For a closer look at how this works, see our article on X-linked recessive inheritance.

• Boys (XY): They have only one X chromosome. If the p21 region on that X chromosome is deleted, there is no second copy to compensate, so the condition always develops.
• Girls (XX): They have two X chromosomes. Even if one X chromosome has the deletion, the other, normal X chromosome can usually compensate, so most girls do not develop symptoms (they become carriers instead). In rare cases, though, mild or even clear symptoms can appear.

Whether or not the mother is a carrier

• Sporadic (de novo) cases: The mother’s genes show no abnormality at all, and the mutation arises for the first time in the child. This accounts for roughly two-thirds of cases. Nothing about diet or lifestyle during pregnancy causes this.
• Inherited cases: The mother is a carrier, and the deletion has been passed down to the child. A son has roughly a 50% chance of inheriting the deletion and developing the condition, and a daughter has roughly a 50% chance of inheriting it, though most daughters remain asymptomatic carriers.

If you are considering a future pregnancy, genetic counseling can provide a more detailed risk assessment.

4. Relationship with Prenatal Diagnosis and NIPT

If NIPT returns a finding related to the Xp21 region, it’s important to understand correctly what that means. Let’s start with how NIPT works in general.

Standard basic NIPT screens for numerical changes in chromosomes 21, 18, and 13. A microdeletion on a sex chromosome only becomes a candidate finding on expanded NIPT plans that include microdeletion and duplication screening. That said, because the extent and breakpoints of the deletion in Xp21 deletion syndrome vary from case to case — unlike the more representative microdeletion syndromes that tend to occur at fixed breakpoints — this condition is harder to design detection for, and whether it’s included on an expanded NIPT panel differs by testing company and plan. Please check the test’s coverage chart before testing to see whether it’s included.

The NIPT guidelines from the Japan Society of Obstetrics and Gynecology state that NIPT results remain within the scope of a screening test and are not a definitive diagnosis. A report from Japan’s Ministry of Health, Labour and Welfare’s expert committee on prenatal testing including NIPT similarly notes that expanded testing, including microdeletion screening, has limits to its analytical and clinical validity. Even if a positive finding comes back, it should not be taken as conclusive — confirmation through chromosomal microarray analysis (CMA) or genetic testing via amniocentesis is required.

If a family already has a member diagnosed with Duchenne muscular dystrophy or congenital adrenal hypoplasia and the causative deletion has already been mapped, a targeted test for that specific deletion via chorionic villus sampling or amniocentesis is more accurate than general NIPT. The fetus’s sex can be one clue when thinking about the risk of an X-linked condition, but a definitive assessment requires genetic counseling and specialized genetic testing. For what NIPT overall can and cannot tell you, see our article on the types of chromosomal abnormalities NIPT can detect; if you’re concerned about microdeletions in general, our article on the risks associated with small chromosomal abnormalities may also help.

NIPT（新型出生前診断）とは｜わかる病気・精度・受けられる時期

NIPT（新型出生前診断）は、妊婦さんの採血から赤ちゃんの染色体疾患の可能性を調べる非確定的検査です。NIPTの仕組み、わかる病気、精度、受けられる時期、注意点をわかりやすく解説します。...

5. Diagnosis and Testing

Several tests are used to confirm the diagnosis and determine exactly how far the deletion extends.

Blood and urine tests

These general tests are usually done first, to pick up signs of the condition.

• CK (creatine kinase) level: A marker of muscle breakdown. When the DMD gene is deleted, this can rise to very high levels (in the thousands to tens of thousands).
• Electrolytes (sodium and potassium): With adrenal insufficiency, sodium tends to run low and potassium tends to run high.
• Glycerol level: Checks whether glycerol concentrations in the blood and urine are markedly elevated.

When elevated CK, electrolyte abnormalities or low blood sugar, and pseudo-hypertriglyceridemia appear together, this syndrome becomes a strong possibility. Any one finding on its own is easy to overlook, so sharing information across departments is often the key to reaching a diagnosis.

Genetic testing

These more detailed tests are used to reach a definitive diagnosis.

• FISH (fluorescence in situ hybridization): Uses a fluorescent probe to confirm whether a specific gene is present.
• Chromosomal microarray analysis (CMA): Can detect deletions from a few hundred kb up to several Mb — far smaller than what conventional microscope-based chromosome analysis (G-banding) can find. It scans the entire chromosome in fine detail to precisely map which genes are missing and which remain.
• MLPA (multiplex ligation-dependent probe amplification): Measures the copy number of specific genes, and is commonly used to define the extent of a DMD gene deletion.

GeneReviews Japan’s overviews of dystrophinopathies and NR0B1-related congenital adrenal hypoplasia both describe an approach combining MLPA with gene panel testing to reach a definitive diagnosis. Testing the mother’s blood alongside the child’s can also determine whether the deletion was inherited from the mother or arose new in the child.

6. Treatment and Management

Current medicine does not yet offer a way to restore the missing genes and cure the condition itself. However, treating each symptom appropriately can prevent complications and maintain or improve quality of life (QOL).

Treatment centers on managing symptoms, with a team spanning several departments — pediatrics, endocrinology, neurology, orthopedics, and rehabilitation medicine — working together.

① Managing adrenal insufficiency (top priority)

This is the single most important area of management for protecting life. The watchwords are: take daily medication, and act quickly on sick days.

• Hormone replacement therapy: Hormones the body cannot make on its own — hydrocortisone and fludrocortisone — are replaced daily with oral medication. This allows the child to live much like any other child.
• Sick-day management: When the body is under stress from fever, vomiting, diarrhea, or injury, it needs more hormone than usual.
  • Take two to three times the usual dose of medication.
  • If the child can’t keep medication down or is vomiting, get to a hospital right away for IV treatment.
  • It’s essential to work with your physician to create an emergency action plan to prevent an “adrenal crisis” (a shock-like state), and share it with family and school.

② Managing muscular dystrophy

• Steroid treatment: May be considered depending on age and condition, and can be expected to help slow the decline in muscle strength in some cases.
• Rehabilitation: Stretching and moderate exercise help prevent joints from stiffening (contractures).
• Cardiac and respiratory monitoring: Regular echocardiograms, ECGs, and pulmonary function tests are performed, with cardioprotective medication started early or respiratory support introduced as needed.

③ Metabolic and nutritional management

• Diet: For glycerol kinase deficiency, avoiding long periods without food and, in some cases, a low-fat diet may be recommended, though how strict this needs to be depends on the individual.
• Weight management: As muscle weakness reduces activity levels, steroid medication can also increase appetite, raising the risk of obesity. Because excess weight places extra strain on muscles and the heart, a well-balanced diet matters.

④ Developmental and educational support

• Early intervention: When language or motor development is delayed, starting physical therapy (PT), occupational therapy (OT), and speech-language therapy (ST) early is recommended.
• School life: The most suitable learning environment — a special-needs class, a resource room, or a special-needs school — is chosen based on the child’s physical and intellectual characteristics. It’s important to clearly communicate to the school how to respond to an adrenal emergency (sick-day management) and precautions around falls.

7. Prognosis

Prognosis varies from person to person, depending on how extensive the deletion is and which symptoms occur together. A delay in managing adrenal insufficiency can be life-threatening, but many cases maintain a good quality of life with early hormone replacement and ongoing multidisciplinary management. Over the long term, how much the muscular dystrophy progresses tends to be a major factor shaping day-to-day life.

It’s difficult to state a uniform prognosis. In our clinical experience, what matters most is regular follow-up and steadily building up preparedness for sick days — taking things one careful step at a time, without rushing.

8. Everyday Life: Points to Keep in Mind and Q&A

Q. Can my child attend a regular school?

A. It depends on the child’s muscle strength, overall condition, and development, but many children do attend regular school. That said, close coordination with the school is needed on the following points.

• Physical education: Strenuous exercise places a strain on muscles, so sitting out or adjusting the activity is often needed.
• Emergencies: A clear system should be in place for immediately contacting parents and for the steps of calling an ambulance in case of fever or injury.
• Getting around: If using stairs is difficult, ask the school about elevator access or adjusting classroom placement.

Q. How should we think about the future?

A. Medical advances continue to improve treatment for Duchenne muscular dystrophy and respiratory management year by year, and gene therapy research is underway around the world. Many adults with this condition use a wheelchair while attending university, working, and enjoying hobbies. While keeping a long-term view, we’d encourage you to focus first on stabilizing “life as it is today.”

Q. Could siblings inherit this?

A. It depends on whether the mother is a carrier. If she is, a son has roughly a 50% chance of being affected, and a daughter has roughly a 50% chance of being a carrier. If the mother is not a carrier (the mutation arose new in the child), the chance of a future child being affected is very low. For an accurate assessment, we recommend consulting a clinical genetics specialist or a certified genetic counselor.

9. Summary

Xp21 deletion syndrome is a complex condition combining multiple symptoms. Here are the key points to remember.

1. Three core issues: This is a “contiguous gene syndrome” combining problems with muscle (DMD), the adrenal glands (AHC), and metabolism (GK).
2. Adrenal management is the lifeline: Taking hormone medication every day and responding quickly on sick days are the most important steps for protecting life.
3. Muscle care: Regular rehabilitation and check-ups aim to preserve function for as long as possible.
4. Where this stands with NIPT: It can come up as a candidate finding on expanded NIPT, but since this isn’t a definitive diagnosis, confirmation via CMA or genetic testing through amniocentesis is required.
5. Team-based care: Many specialists are involved. Don’t hesitate to ask your doctors and care team any questions you have.

10. A Message to Families Who Have Received This Diagnosis

Hearing the diagnosis “Xp21 deletion syndrome,” you may right now be feeling deep sadness, anxiety, or an unanswerable “why my child?” You may have searched online, found mostly difficult information, and felt overwhelmed.

Please don’t blame yourself. This condition is never caused by how parents raise a child or by anything that happened during pregnancy.

This condition does call for lifelong management, but with appropriate treatment, your child can smile, grow, and share irreplaceable time with your family. Medicine keeps advancing, and new treatments continue to be developed.

Because this is a rare condition, you may not know anyone nearby facing the same situation, which can feel isolating. But support resources do exist — family associations for people with the same condition, and rare-disease support and consultation centers, among others. In Japan, Duchenne muscular dystrophy is recognized as a designated intractable disease, and congenital adrenal hypoplasia is recognized as a designated intractable disease and pediatric chronic specific disease, both of which can qualify for public medical-expense assistance.

• Working with specialists: Your child’s doctor is your partner. Please bring up even small concerns.
• Family and patient associations: Patient groups for Duchenne muscular dystrophy or congenital adrenal hypoplasia sometimes include families affected by Xp21 deletion syndrome as well. Hearing from families further along this path can be a great source of support.
• Public support: Programs for pediatric chronic specific diseases, designated intractable diseases, and disability certification can all help with medical costs and welfare services. Ask your hospital’s medical social worker for guidance.

You don’t need to understand everything today. Take it one day at a time, watching how your child is doing, and walk this path together with your medical team. We’re rooting for you — for a future that is calm and full of hope, for you and your child.

If you’re considering NIPT for a future child, how far the microdeletion coverage extends will shape which plan is right for you. If you’re not sure which plan fits your situation, try our Plan Finder tool. We also welcome phone inquiries (Japanese-language line): 0120-169-629.

If you’d like to understand the basics of how inheritance works, our article on the fundamentals of genetic inheritance may also help.

Frequently Asked Questions

What is Xp21 deletion syndrome?

It’s a contiguous gene syndrome that occurs when a region on the short arm of the X chromosome, at Xp21, is congenitally missing across a stretch of DNA. When the three genes DMD, GK, and NR0B1 are deleted together, symptoms affecting muscle, adrenal function, and metabolism overlap in the same person.

Why do muscle weakness, adrenal insufficiency, and metabolic abnormalities happen at the same time?

Because the DMD, GK, and NR0B1 genes sit right next to each other on the X chromosome. When this region is deleted as a block, the conditions associated with each individual gene develop together. The larger the deletion, the more genes tend to be caught up in it.

What is the inheritance pattern, and could it affect a future child?

It follows an X-linked recessive inheritance pattern. If the mother is a carrier, a son has roughly a 50% chance of developing the condition, and a daughter has roughly a 50% chance of becoming a carrier. Because it can also arise as a new mutation, confirmation through genetic counseling is recommended.

Can NIPT detect this condition?

It can come up as a candidate finding on expanded NIPT plans that include microdeletion screening, but because the extent of the deletion varies case by case, whether it’s covered depends on the testing company and plan. Even a positive finding is not a definitive diagnosis — confirmation via CMA or genetic testing through amniocentesis is needed.

How is a definitive diagnosis made?

Based on biochemical findings such as CK levels, electrolyte abnormalities, and pseudo-hypertriglyceridemia, chromosomal microarray analysis (CMA) or a gene panel test is used to map the extent of the deletion. The mother’s blood is typically tested alongside the child’s to determine whether the deletion was inherited or arose new.

Is there a treatment?

There is no cure yet, but hormone replacement therapy for adrenal insufficiency takes top priority. Alongside it, care combines rehabilitation for muscular dystrophy, cardiac and respiratory monitoring, metabolic management, and developmental support.

Medical supervision: Dr. Hiroshi Oka — Director-in-Chief and Lab Director, Hiro Clinic (Fukumikai Medical Corporation). A graduate of Keio University School of Medicine, Dr. Oka holds a Ph.D. in Medicine and has passed the national medical licensing examinations of both Japan and the United States. He is one of the few physicians in Japan to hold laboratory director credentials. This article was prepared in line with Japan’s medical advertising guidelines, drawing on public and academic sources including GeneReviews Japan, the Japan Intractable Disease Information Center, the Center for Pediatric Chronic Disease Information, the Japan Society of Obstetrics and Gynecology, and Japan’s Ministry of Health, Labour and Welfare. Reported frequencies and figures vary somewhat between sources; please consult your physician for decisions about diagnosis and treatment.

References

• Xp21 contiguous gene deletion syndrome presenting as Duchenne muscular dystrophy and glycerol kinase deficiency associated with intellectual disability: case report and review literature. PMC.
• Xp21 Contiguous Gene Deletion Syndrome: Diagnosis, Treatment, and a Review of the Literature on a Rare Genetic Disorder. Journal of Clinical Research in Pediatric Endocrinology, 2025.
• GeneReviews Japan. NR0B1-Related Congenital Adrenal Hypoplasia.
• GeneReviews Japan. Dystrophinopathies.
• Dystrophinopathies. GeneReviews®. University of Washington, Seattle.
• X-Linked Adrenal Hypoplasia Congenita. GeneReviews®. University of Washington, Seattle.
• Center for Pediatric Chronic Disease Information. Congenital Adrenal Hypoplasia — Overview.
• Japan Intractable Disease Information Center. Congenital Adrenal Hypoplasia (Designated Intractable Disease 82).
• Japan Intractable Disease Information Center. Muscular Dystrophy (Designated Intractable Disease 113).
• Japan Society of Obstetrics and Gynecology. Guidelines on NIPT.
• Ministry of Health, Labour and Welfare. Report of the Expert Committee on Prenatal Testing Including NIPT.