Williams syndrome is caused by a missing stretch of DNA on chromosome 7. Specifically, about 25 genes are deleted from a region known as 7q11.23, creating a gap of roughly 1.5 to 1.8 million base pairs. This deletion happens spontaneously in most cases, meaning it isn’t inherited from a parent. The condition affects approximately 1 in 10,000 live births.
The Deletion on Chromosome 7
Every cell in your body carries two copies of chromosome 7, one from each parent. In Williams syndrome, a small but significant chunk of one copy is missing. That missing piece contains around 25 genes, many of which play roles in brain development, connective tissue structure, and calcium regulation. Because only one copy of each gene remains functional, cells produce roughly half the normal amount of each protein those genes code for. That partial loss is enough to cause the wide range of features seen in the syndrome.
The technical term for this type of genetic change is a hemizygous microdeletion. “Micro” because the deleted segment is too small to see under a standard microscope, and “hemizygous” because only one of the two gene copies is lost. The deletion occurs during the formation of a sperm or egg cell, which is why it almost always appears in families with no prior history of the condition.
Why It’s Almost Never Inherited
In the vast majority of cases, the deletion arises de novo, meaning it is a new, random event. Neither parent carries the deletion or did anything to cause it. There is no known environmental trigger, dietary factor, or lifestyle choice that increases risk. The region of chromosome 7 where the deletion occurs is flanked by repetitive DNA sequences that make it structurally prone to errors when chromosomes exchange material during cell division.
If a parent does carry the deletion (meaning the parent has Williams syndrome), each of their children has a 50% chance of inheriting it. This follows a standard autosomal dominant pattern. But because the deletion is spontaneous in most families, the recurrence risk for parents who don’t have the syndrome and already have one affected child is very low.
How Missing Genes Produce Specific Symptoms
Not all 25 deleted genes contribute equally to the syndrome’s features. Researchers have linked several individual genes to specific characteristics, which helps explain why Williams syndrome affects the heart, brain, personality, and connective tissues all at once.
Elastin and Heart Problems
The gene with the most clearly understood role is ELN, which provides instructions for making elastin, a protein that gives blood vessels, skin, lungs, and ligaments their stretch and resilience. With only one working copy, the body produces about half the normal amount of elastin. The result is blood vessels that are thicker, stiffer, and less flexible than they should be.
This elastin shortage is directly responsible for the most common heart defect in Williams syndrome: a narrowing of the aorta just above the heart valve, called supravalvular aortic stenosis. It occurs in roughly 45% of affected individuals. Another frequent finding is narrowing of the pulmonary arteries, present in about 37%. Both conditions restrict blood flow and can require surgical repair, though most people with Williams syndrome do not need heart surgery during long-term follow-up, and overall mortality from cardiovascular complications has been low in large studies.
Beyond the heart, reduced elastin also explains the characteristically soft skin, loose joints, and occasional mild lung problems seen in the condition.
Sociability and Visuospatial Challenges
Williams syndrome produces a distinctive cognitive and personality profile: unusually high sociability, a strong draw toward other people (sometimes described as “hypersociality”), relatively strong language skills, but notable difficulty with spatial tasks like assembling puzzles or judging distances. Research has traced much of the social personality to a gene called GTF2I. Studies in mice show that having only one copy of this gene leads to higher levels of less-selective social interaction, mirroring what’s observed in people with the syndrome.
Two other deleted genes, LIMK1 and CLIP2, are involved in how nerve cells migrate and mature during brain development. Their loss likely contributes to the visuospatial difficulties and mild to moderate intellectual disability that typically accompany the condition. The interplay among multiple missing genes, rather than any single deletion, is what produces the full behavioral and cognitive picture.
Elevated Calcium Levels
Many infants with Williams syndrome experience hypercalcemia, or abnormally high calcium in the blood. The exact mechanism is still not fully understood, but it appears to involve a combination of increased calcium absorption in the gut (possibly driven by elevated levels of active vitamin D) and reduced ability of the kidneys to clear excess calcium. This typically improves with age but can cause irritability, feeding difficulties, and discomfort in early infancy.
How the Deletion Is Detected
Because the missing segment is too small to appear on a standard chromosome test (called a karyotype), Williams syndrome requires more sensitive methods. Chromosomal microarray analysis is now the recommended first-line genetic test for children with unexplained developmental delays or congenital anomalies. It can detect deletions as small as a few hundred thousand base pairs, giving it more than ten times the resolution of traditional karyotyping. An older test called FISH (fluorescence in situ hybridization) can also identify the specific 7q11.23 deletion and is still sometimes used when Williams syndrome is strongly suspected based on clinical features.
Diagnosis often begins with a pediatrician or cardiologist recognizing the combination of a characteristic facial appearance, heart defects, and developmental patterns. The genetic test then confirms the deletion.
What This Means for Affected Families
Understanding the genetic cause provides concrete answers. The deletion is a one-time biological event, not the result of anything preventable. For parents who already have one child with Williams syndrome and do not carry the deletion themselves, the chance of having another affected child is extremely small. For individuals with Williams syndrome who have children of their own, each pregnancy carries a 50% chance of passing on the deletion.
Cardiovascular abnormalities are present in roughly 82% of people with Williams syndrome, making regular cardiac monitoring the most important aspect of ongoing medical care. Interventions, when needed, typically happen before age 5. With appropriate surveillance and support, most individuals live well into adulthood, and the overall prognosis has improved significantly as awareness and monitoring have increased.