A microdeletion syndrome is a condition caused by a small missing piece of a chromosome. Unlike large chromosomal abnormalities (such as Down syndrome, where an entire extra chromosome is present), microdeletions involve losing a stretch of DNA too small to see under a standard microscope, typically ranging from a few hundred thousand to several million base pairs. That missing segment can contain dozens of genes, and the loss of those genes produces a recognizable pattern of physical, developmental, and behavioral features.
How Microdeletions Happen
Your DNA is packaged into 23 pairs of chromosomes, and each chromosome contains thousands of genes arranged in a specific order. A microdeletion occurs when a small segment of one chromosome breaks away and is lost during cell division, either when sperm or egg cells form or very early in embryonic development. The result is a child who has one working copy of every gene in that region instead of the usual two.
For most genes, a single copy is enough to function normally. But certain genes are “dose-sensitive,” meaning the body needs both copies to develop and operate correctly. When one copy is missing, the reduced output of those genes causes the specific symptoms associated with that syndrome. The exact combination of genes lost determines which syndrome a person has and which body systems are affected.
Whether a microdeletion is inherited or happens spontaneously depends on the specific syndrome. Some occur almost entirely as new (de novo) events, meaning neither parent carries the deletion. Others are frequently inherited. For the 15q13.3 deletion, for example, roughly 85% of cases are inherited from a parent who may have mild or no obvious symptoms, while only about 15% arise spontaneously. This variability makes genetic counseling an important part of diagnosis.
The Most Common Microdeletion Syndromes
The five most frequently diagnosed microdeletion syndromes are:
- 22q11.2 deletion syndrome (DiGeorge syndrome): the most common of all, affecting roughly 1 in 6,000 live births in the United States. It involves heart defects, immune system problems, speech delays, and learning difficulties.
- Prader-Willi syndrome (15q11-q13): characterized by low muscle tone in infancy, followed by excessive hunger and obesity risk in childhood, along with intellectual disability and behavioral challenges.
- Angelman syndrome (15q11-q13): causes severe developmental delay, limited speech, seizures, and a characteristically happy demeanor with frequent smiling and laughter.
- Williams syndrome (7q11.23): associated with heart and blood vessel problems, distinctive facial features, strong verbal abilities, and an unusually outgoing personality.
- Wolf-Hirschhorn syndrome (4p16.3): involves growth delay, intellectual disability, seizures, and distinctive facial features sometimes described as a “Greek warrior helmet” appearance.
When the Same Deletion Causes Different Syndromes
One of the more striking aspects of microdeletion genetics is that a deletion in the same chromosomal region can produce entirely different conditions depending on which parent the affected chromosome came from. Prader-Willi syndrome and Angelman syndrome both involve deletions at the same location on chromosome 15, but they look nothing alike.
This happens because of a phenomenon called genomic imprinting. Certain genes are chemically tagged so that only the copy from one parent is active. If the deletion removes the father’s copy of chromosome 15q11-q13, the child develops Prader-Willi syndrome, because the genes that needed to come from the father are gone. If the deletion removes the mother’s copy, the child develops Angelman syndrome, because a different gene (one that’s only active on the maternal chromosome) is lost. The same missing DNA, two completely different outcomes, determined solely by parental origin.
Williams Syndrome as a Case Study
Williams syndrome illustrates how losing a small cluster of genes can affect multiple body systems at once. The deletion removes 25 to 27 genes from chromosome 7, and researchers have traced specific symptoms to specific missing genes. Loss of the ELN gene, which provides instructions for making a protein called elastin, leads to connective tissue problems and a narrowing of the aorta (the large blood vessel leaving the heart) called supravalvular aortic stenosis. Narrowing can also occur in the blood vessels supplying the lungs and the heart itself, and high blood pressure is common.
Other deleted genes in the same region account for the cognitive and behavioral profile. Children with Williams syndrome typically struggle with visual-spatial tasks like drawing or assembling puzzles, yet they often have strong spoken language skills and an aptitude for music and rote memorization. They tend to be remarkably social, showing an intense interest in other people, though anxiety disorders and attention difficulties are also common. This mix of strengths and challenges, all traced to one small chromosomal gap, shows why microdeletion syndromes affect so many aspects of a person’s life simultaneously.
How Microdeletions Are Detected
Standard chromosome testing (karyotyping) examines chromosomes under a microscope and can only reliably detect missing pieces larger than about 10 to 20 million base pairs. Most microdeletions fall well below that threshold, which is why they went undiagnosed for decades and why many are still missed when only basic testing is performed.
Two more advanced methods have largely replaced karyotyping for these conditions. FISH (fluorescence in situ hybridization) uses fluorescent probes that bind to a specific chromosomal region. If the probe fails to light up on one chromosome, the deletion is confirmed. FISH targets imbalances as small as 100 to 200 kilobases, but it only checks the specific location you ask it to check. A clinician needs to already suspect a particular syndrome to order the right probe.
Chromosomal microarray analysis (CMA) is now the preferred first-line test. It scans the entire genome in a single run and can detect deletions and duplications down to the kilobase range, hundreds of times smaller than what a standard karyotype reveals. Because it doesn’t require a clinical suspicion to guide it, CMA can identify unexpected microdeletions that wouldn’t have been caught otherwise.
Prenatal Screening
Non-invasive prenatal testing (NIPT), which analyzes fragments of fetal DNA circulating in the mother’s blood, can now screen for some microdeletions during pregnancy. However, accuracy is considerably lower than for whole-chromosome conditions like Down syndrome. A large study of over 68,000 pregnancies found that the positive predictive value for microdeletions was about 56%, meaning that nearly half of positive screening results turned out to be false alarms after confirmatory testing. Because of this, a positive NIPT result for a microdeletion is treated as a screening flag, not a diagnosis, and is always followed up with CMA on an amniocentesis or chorionic villus sampling to confirm.
Living With a Microdeletion Syndrome
There is no way to replace the missing DNA, so management focuses on identifying and treating each affected body system as early as possible. Because microdeletions typically involve many genes, children often need care from a wide range of specialists. A dedicated clinic for 22q11.2 deletion syndrome at one Florida community hospital, for instance, grew from an initial team of six specialties (genetics, immunology, ear-nose-and-throat, endocrinology, speech pathology, and nutrition) to 14 providers spanning cardiology, neurology, psychology, nephrology, pulmonology, social work, and more. Even then, patients frequently needed additional support from psychiatry, gastroenterology, dental, and orthopedic specialists.
This kind of coordinated, multidisciplinary approach makes a meaningful difference in outcomes. Early cardiac screening can catch heart defects before they become dangerous. Speech therapy started in infancy helps children develop communication skills during critical developmental windows. Behavioral support and educational accommodations can build on a child’s strengths while addressing specific learning challenges. The breadth of care needed varies by syndrome and by individual, since even people with the same deletion can differ significantly in which features they develop and how severely.
Many people with microdeletion syndromes live into adulthood and benefit from ongoing monitoring. Heart and blood vessel issues may need lifelong follow-up. Mental health support becomes increasingly important through adolescence, when anxiety and social challenges often intensify. Transition planning from pediatric to adult care is a practical concern that families typically begin addressing in the mid-teen years, ensuring that the network of specialists a child relied on doesn’t disappear at age 18.