Jacobsen syndrome is a rare genetic condition caused by a deletion on the long arm of chromosome 11, typically removing a stretch of DNA that contains dozens of genes at once. First described in 1973 by the Danish geneticist Petrea Jacobsen, the syndrome affects multiple organ systems and leads to a wide range of features including bleeding problems, heart defects, distinctive facial characteristics, and intellectual disability. Fewer than 200 to 300 cases have been documented in the medical literature, and the condition’s rarity means that even experienced clinicians may encounter it only once or twice in a career.
What Is Missing on Chromosome 11
The deletion that causes Jacobsen syndrome sits at the far end of chromosome 11’s long arm, a region labeled 11q23 through 11q25. The missing piece ranges from roughly 7 million to 20 million DNA base pairs, which is a substantial chunk of genetic real estate containing many genes that play roles in blood cell production, heart development, brain wiring, and immune function.1PubMed Central. Jacobsen syndrome In most cases, the breakpoint falls within or just beyond a sub-region called 11q23.3, and the deletion extends all the way to the tip of the chromosome. A minority of patients have a breakpoint at a known fragile site on the chromosome called FRA11B, a spot in the DNA that is prone to breaking during cell division.
Most deletions happen spontaneously, arising as a new event in a parent’s egg or sperm cell or very early in embryonic development. Occasionally, a parent carries a balanced chromosomal rearrangement that produces no symptoms in them but can result in an unbalanced deletion when passed to a child. A case report published in 2024 documented the first instance of Jacobsen syndrome caused by chromoanasynthesis, an unusually complex rearrangement in which multiple duplications and deletions occurred simultaneously on one copy of chromosome 11q during a single catastrophic event in a reproductive cell.2PubMed Central. Chromoanasynthesis as a cause of Jacobsen syndrome This finding expanded the known ways the syndrome can originate beyond simple terminal deletions and balanced-translocation inheritance.
The Genes That Drive the Key Features
Because the deletion spans many genes, Jacobsen syndrome is classified as a contiguous gene disorder, meaning that the clinical picture arises from losing one working copy of numerous genes at once rather than a single gene mutation. Over the past two decades, researchers have narrowed down which genes matter most for specific features.
Two genes stand out: FLI1 and ETS1. A study of two families with unusually small deletions, just 700 thousand base pairs, found that a deletion containing only FLI1, ETS1, and a long noncoding RNA called SENCR was enough to produce several hallmark features of the syndrome, including low platelet counts, heart defects, immune problems, and ear and nose abnormalities.3PubMed. 11q24.2q24.3 microdeletion in two families presenting features of Jacobsen syndrome, without intellectual disability: Role of FLI1, ETS1, and SENCR long noncoding RNA Mice engineered to carry only one working copy of both Ets1 and Fli1 developed craniofacial changes, hearing loss, middle-ear infections, and a small middle-ear cavity, closely mimicking what is seen in children with the syndrome.4PubMed. Mice Haploinsufficient for Ets1 and Fli1 Display Middle Ear Abnormalities and Model Aspects of Jacobsen Syndrome The mouse studies also showed that the two genes partially back each other up: losing one copy of Fli1 alone caused mild problems, but losing one copy of both genes together produced more severe hearing impairment and worse bone abnormalities in the ear’s tiny bones.
FLI1 is the primary driver of the bleeding disorder (discussed below), while ETS1 appears to be most important for heart development and immune-cell maturation. The noncoding RNA SENCR, which sits in the same small region, is thought to play a role in limb defects because of its involvement in blood-vessel cell development, though this connection is still being investigated.5PubMed. 11q24.2q24.3 microdeletion in two families presenting features of Jacobsen syndrome, without intellectual disability: Role of FLI1, ETS1, and SENCR long noncoding RNA Beyond these three, the deletion can encompass dozens of additional genes, including those involved in brain myelination and immune-cell signaling, which helps explain why the clinical picture varies so widely from one patient to the next depending on exactly how much DNA is missing.
The Bleeding Problem and Paris-Trousseau Syndrome
Almost all children with Jacobsen syndrome have some form of platelet abnormality. Estimates suggest that roughly 95% of reported patients are affected, and the problem involves not just the number of platelets but also their size and how well they function.6Paediatric anaesthesia. Anesthetic implications of Jacobsen syndrome The specific platelet disorder associated with the syndrome is called Paris-Trousseau thrombocytopenia. Under an electron microscope, the platelets contain abnormally large storage granules, a distinctive finding that helped researchers realize Paris-Trousseau syndrome and Jacobsen syndrome were related conditions rather than separate entities.7PubMed. Paris-Trousseau syndrome platelets in a child with Jacobsen’s syndrome
The root cause is loss of one copy of FLI1, which encodes a protein that helps immature blood cells develop into properly functioning platelets. With only one working copy, the bone marrow produces platelets that are fewer in number and functionally impaired. A study confirmed this link by identifying a family without any chromosome 11 deletion who nonetheless had Paris-Trousseau-like bleeding because both copies of their FLI1 gene carried disabling mutations, essentially phenocopying the deletion’s effect through a different genetic route.8PubMed. Paris-Trousseau thrombocytopenia is phenocopied by the autosomal recessive inheritance of a DNA-binding domain mutation in FLI1
For families, the practical impact is that bruising and prolonged bleeding are common from early childhood. Minor procedures like dental work or ear-tube placement carry higher risk, and platelet transfusions may be needed before surgery. The platelet dysfunction does not respond to typical treatments for other low-platelet conditions, so management centers on awareness and preparation rather than medication.
Heart Defects
Congenital heart defects occur in a large share of children with Jacobsen syndrome, and the specific types tend to cluster around problems with the structures that form the inner walls and valves of the heart. One of the most serious is hypoplastic left heart syndrome, in which the left side of the heart is severely underdeveloped and cannot pump blood to the body effectively. A case report described a patient with Jacobsen syndrome and hypoplastic left heart who eventually required a heart transplant, illustrating the severity of cardiac involvement at the far end of the spectrum.9PubMed Central. Jacobsen Syndrome with Hypoplastic Left Heart Syndrome: Outcome after Cardiac Transplantation Endocardial cushion defects, which affect the central portion of the heart where the chambers and valves meet, are also reported frequently.10Journal of Medical Genetics. Jacobsen syndrome: report of a patient with severe eye anomalies, growth hormone deficiency, and hypothyroidism associated with deletion 11 (q23q25) and review of 52 cases
Because heart problems can be life-threatening in the newborn period, early cardiac imaging is a standard part of care for any infant diagnosed with a distal 11q deletion. Surgical correction is possible for many of these defects, but the combination of heart disease and platelet dysfunction makes perioperative management especially delicate.
Facial Features, Eyes, and Hearing
Children with Jacobsen syndrome often share a recognizable set of craniofacial features. These commonly include a skull shape called trigonocephaly, in which the forehead narrows to a point at the midline, along with widely spaced eyes, downward-slanting eyelid openings, and a short nose.11Journal of Medical Genetics. Jacobsen syndrome: report of a patient with severe eye anomalies, growth hormone deficiency, and hypothyroidism associated with deletion 11 (q23q25) and review of 52 cases As discussed earlier, the mouse studies demonstrated that these nasal and ear-related features are largely attributable to the combined loss of ETS1 and FLI1.
Eye problems are extremely common. In one review, 90% of patients had strabismus (misaligned eyes) and 90% had significant refractive errors requiring glasses. Drooping eyelids affected about 70%. Less common findings included abnormally tortuous retinal blood vessels, glaucoma, and amblyopia.12PubMed. Ocular findings in Jacobsen syndrome Some patients also have colobomas, which are gaps in the structures of the eye that form when the eye does not close completely during fetal development. These can affect the iris, the choroid layer, or the retina, and in severe cases they impair vision significantly.
Hearing loss is another frequent concern, driven both by structural differences in the middle ear and by chronic ear infections. The mouse models showed that loss of one copy of Fli1 leads to malformed stapes (one of the three tiny bones that transmit sound in the middle ear) and a small middle-ear cavity, both of which reduce sound transmission.13PubMed. Mice Haploinsufficient for Ets1 and Fli1 Display Middle Ear Abnormalities and Model Aspects of Jacobsen Syndrome Chronic middle-ear fluid and infection compound the problem. Early hearing screening and prompt management of ear infections are crucial for supporting language development.
Brain Development and White Matter Changes
Brain MRI in children with Jacobsen syndrome sometimes reveals white matter abnormalities, areas where the insulating coating on nerve fibers appears delayed or atypical. One case documented diffuse abnormal white-matter signals at 9 months of age, with follow-up imaging at 2.5 years showing that the white matter had progressively matured and the abnormalities had largely resolved.14PubMed. White matter abnormality in Jacobsen syndrome assessed by serial MRI A separate case report found a similar pattern: unmyelinated white matter in the parieto-occipital lobes at initial imaging, followed by significant improvement ten months later that correlated with better developmental and motor outcomes.15PubMed Central. Jacobsen Syndrome With White Matter Abnormalities: A Case Report and MRI Follow-Up These white matter changes are believed to involve swelling within the myelin sheath linked to deletion of a gene called HEPACAM (also known as GlialCAM).
The encouraging finding is that these white matter abnormalities can improve over time, which may partly explain why some children show developmental gains in motor and cognitive skills as they grow. Still, structural brain anomalies beyond delayed myelination have been reported in some patients, including thickened brain folds, abnormal cell clusters lining the ventricles, enlarged ventricles, underdevelopment of the cerebellar vermis, and absence of the corpus callosum (the bridge connecting the two brain hemispheres).16American Journal of Neuroradiology. Spectrum of Clinical and Associated MR Imaging Findings in Children with Olfactory Anomalies The range of brain findings is wide, and not every child will have visible structural differences on imaging.
Intellectual Disability, Autism, and Behavior
Intellectual disability is part of the syndrome for the majority of affected individuals, though the degree varies considerably. A 2025 study that performed detailed neuropsychological testing on a cohort of patients found a mean IQ of about 50, placing most individuals in the moderate range of intellectual disability. The range of adaptive functioning was very broad, meaning some people managed daily tasks with modest support while others needed much more help. About 60% of participants had verbal language.17PubMed Central. Clinical and neuropsychological characterization of Jacobsen syndrome (del11q) The same study found that larger deletions were associated with more delayed developmental milestones and lower cognitive scores, which makes intuitive sense given that bigger deletions remove more genes.
Autism spectrum features are strikingly common. In a study that used standardized diagnostic instruments, about 47% of individuals with Jacobsen syndrome met research criteria for an autism spectrum diagnosis. The rate was higher in males (80% of those who completed testing) than in females (33%).18Genetics in Medicine. Evidence for autism spectrum disorder in Jacobsen syndrome: identification of a candidate gene in distal 11q The 2025 study reported a broadly similar figure of 43% exceeding an autism screening cutoff.19PubMed Central. Clinical and neuropsychological characterization of Jacobsen syndrome (del11q) Interestingly, the earlier study found no clear correlation between deletion size and autism, suggesting that the autism risk is driven by specific genes in the deleted region rather than the overall amount of missing DNA.
One candidate gene for the autism link is PX-RICS (also known as ARHGAP32), which sits in the commonly deleted region. Mice lacking this gene showed social behavior changes resembling autism and had reduced levels of a key inhibitory receptor on their brain cells, disrupting the balance between excitatory and inhibitory signaling.20Nature Communications. PX-RICS-deficient mice mimic autism spectrum disorder in Jacobsen syndrome through impaired GABAA receptor trafficking However, the 2025 neuropsychological study did not find a significant association between losing one copy of PX-RICS (ARHGAP32) and either cognitive functioning or autistic characteristics in human patients, leaving the gene’s role in real-world outcomes uncertain.21PubMed Central. Clinical and neuropsychological characterization of Jacobsen syndrome (del11q) This disconnect between animal models and human cohorts is a recurring theme in rare-disease research: a gene can look like the obvious culprit in mice but behave differently against the complex genetic backdrop of a human deletion.
An unexpected finding from the 2025 study was that individuals with higher cognitive ability were more likely to experience depressive symptoms or receive a diagnosis of depression, a pattern sometimes seen in other intellectual-disability syndromes where greater awareness of one’s own differences can carry emotional weight.22PubMed Central. Clinical and neuropsychological characterization of Jacobsen syndrome (del11q)
Immune System Vulnerability
Some children with Jacobsen syndrome experience recurrent infections that go beyond what you would expect from chronic ear disease alone, pointing to an underlying immune deficiency. A molecular study of one patient identified four deleted genes as likely contributors to the immune problems: FLI-1, ETS1, JAM3, and THYN1.23PubMed Central. Immune Deficiency in Jacobsen Syndrome: Molecular and Phenotypic Characterization ETS1 is the most studied of these in the context of immune function; it plays a central role in the development and differentiation of several types of immune cells. The practical implication is that children with Jacobsen syndrome may benefit from immunological workups if they seem to get sick more often or more severely than expected, and their care teams should have a low threshold for investigating immune function.
Prenatal and Postnatal Diagnosis
Jacobsen syndrome can sometimes be suspected before birth when routine ultrasound reveals certain patterns of abnormalities, such as heart defects combined with growth restriction or other structural anomalies. Amniocentesis followed by chromosomal microarray analysis can confirm the diagnosis by detecting the specific deletion and defining its size. One prenatal case was identified this way after ultrasound findings prompted genetic testing, revealing both a duplication and a large deletion on chromosome 11q.24PubMed Central. Ultrasonographic findings and prenatal diagnosis of Jacobsen syndrome: A case report and review of the literature Microarray-based methods have proven especially useful because standard chromosome banding (karyotyping) can miss smaller or more complex rearrangements, while microarray analysis detects both deletions and duplications with much finer resolution.25PubMed. Prenatal diagnosis of Jacobsen syndrome associated with a distal 11q deletion and a distal 8q duplication by chromosome microarray analysis in a fetus with a de novo unbalanced translocation
After birth, the combination of low platelet counts, distinctive facial features, and congenital heart disease in a newborn should raise suspicion for a distal 11q deletion. Chromosomal microarray is now the standard confirmatory test. Knowing the exact size and location of the deletion is clinically useful because it helps predict which features are most likely, although there is always individual variability.
Anesthesia and Surgical Planning
Children with Jacobsen syndrome often need surgical procedures, whether for heart repair, ear tubes, eye corrections, or other issues. The combination of airway challenges and platelet dysfunction makes anesthesia planning particularly important. Features like a short neck, small jaw, and unusual mouth shape can make intubation more difficult. Critically, the platelet problem needs to be managed before and during any procedure, as standard coagulation labs may look deceptively normal while the platelets themselves are functionally impaired.26Paediatric anaesthesia. Anesthetic implications of Jacobsen syndrome Surgeons and anesthesiologists unfamiliar with the syndrome may not anticipate this combination of risks, which is why families and primary care teams often prepare detailed medical summaries to share before any procedure.
How Deletion Size Shapes the Clinical Picture
One of the more informative findings from the genetic literature is how much the clinical picture depends on exactly which genes are deleted. The two families with the smallest known deletions (about 700 thousand base pairs, losing only FLI1, ETS1, and SENCR) had thrombocytopenia, heart defects, immune problems, and facial features, but no intellectual disability.27PubMed. 11q24.2q24.3 microdeletion in two families presenting features of Jacobsen syndrome, without intellectual disability: Role of FLI1, ETS1, and SENCR long noncoding RNA This tells us that the cognitive impairment seen in most patients comes from genes located outside this tiny critical region, further toward the center of the chromosome, which are lost only in larger deletions.
The 2025 neuropsychological study confirmed this gradient: patients with larger deletions had more delayed milestones and lower cognitive scores.28PubMed Central. Clinical and neuropsychological characterization of Jacobsen syndrome (del11q) Yet autism did not follow the same pattern; some individuals with relatively small deletions met autism criteria while some with large deletions did not.29Genetics in Medicine. Evidence for autism spectrum disorder in Jacobsen syndrome: identification of a candidate gene in distal 11q The implication is that different clinical features are governed by different genes within the deletion, and the relationship between deletion size and outcome is not a simple “bigger deletion equals worse prognosis” equation across every domain. Knowing the specific breakpoints can give families and clinicians a better sense of which complications are most likely, even though individual variability always remains.
Endocrine and Growth Issues
Growth retardation is common in Jacobsen syndrome and can be present from before birth. Some patients have documented growth hormone deficiency, and at least one case was reported with both growth hormone deficiency and hypothyroidism.30Journal of Medical Genetics. Jacobsen syndrome: report of a patient with severe eye anomalies, growth hormone deficiency, and hypothyroidism associated with deletion 11 (q23q25) and review of 52 cases Screening for hormone-related growth problems is worth pursuing when a child with the syndrome is growing more slowly than expected, as growth hormone replacement can be effective when a genuine deficiency is identified. Feeding difficulties in infancy, related to low muscle tone and oral-motor coordination problems, can also contribute to poor weight gain independently of any hormonal cause.
Living with Jacobsen Syndrome
For patients who survive the newborn period and infancy, when heart defects and severe bleeding pose the greatest immediate risks, the long-term trajectory varies enormously. Life expectancy for those past early childhood remains uncertain in the medical literature, in part because the syndrome is so rare that large, long-term follow-up studies have not been conducted.31The Turkish Journal of Pediatrics. Jacobsen syndrome without thrombocytopenia: a case report and review of the literature Many individuals reach adulthood, but they typically need ongoing support for medical monitoring, educational needs, and daily living. Speech therapy, occupational therapy, and behavioral support are mainstays of care throughout childhood, and the wide variation in cognitive ability means that educational and vocational planning needs to be individualized rather than assumed from the diagnosis alone.
Since the original description by Petrea Jacobsen in 1973, more than 200 patients have been reported in the literature.32PubMed. Jacobsen syndrome: Advances in our knowledge of phenotype and genotype The advent of microarray testing has likely increased detection, particularly of smaller or atypical deletions that would have been missed by older methods. As more patients are identified and followed, the understanding of the syndrome’s natural history continues to sharpen, giving families and clinicians better tools for anticipating needs and planning care.