The Disorder

A disorder named for what it disturbs, not what it predicts.

Heterozygous variants in DLL1 cause a highly variable neurodevelopmental disorder — sometimes called DLL1-related neurodevelopmental disorder, more fully "neurodevelopmental disorder with non-specific brain abnormalities and/or vertebral malformations." Inside that careful name is a wide range of human experience.

At a glance

Mechanism
Haploinsufficiency
OMIM
#618709
First described
2019
Inheritance
Autosomal dominant · mostly de novo
Affected systems
CNS · skeletal · behavioral
Reported cases
Few dozen and growing

Features cluster around the systems Notch helps build

DLL1 is most active during development of the brain, the vertebral column, and the immune system. That is exactly where the disorder shows up.

01Cognitive

Intellectual & developmental features

The most consistently reported feature. Severity varies widely — from mild learning differences to substantial intellectual disability with significant support needs.

  • Global developmental delay in infancy and early childhood
  • Intellectual disability across a wide severity range
  • Speech and language delay, sometimes with limited or no spoken language
  • Difficulty with abstract reasoning or executive function
02Behavioral

Autism spectrum & behavioral features

Many — though not all — affected individuals meet criteria for autism spectrum disorder. Other neurobehavioral features may co-occur.

  • Autism spectrum disorder or autistic features
  • Attention difficulties, hyperactivity, or impulsivity
  • Anxiety, sensory sensitivities, or rigid routines
  • Sleep disturbances
03Neurological

Brain structure & neurological features

The "non-specific brain abnormalities" in the disorder’s name. MRI findings vary; there is no single signature lesion. Seizures occur in a subset.

  • Variable MRI findings — thin corpus callosum, ventricular enlargement, white-matter changes
  • Some individuals have structurally normal-appearing brain imaging
  • Seizures (epilepsy) in a subset of cases
  • Hypotonia, particularly in infancy
  • Motor coordination difficulties
04Skeletal

Vertebral & skeletal features

Notch signaling is essential for somitogenesis — how the early embryo segments itself into the building blocks of the spine. DLL1 disruption can leave fingerprints there, though not in every individual.

  • Vertebral malformations — hemivertebrae, butterfly vertebrae, fused vertebrae
  • Scoliosis or other spinal curvature
  • Rib anomalies in some individuals
  • Joint hypermobility or contractures reported variably
  • Not all affected individuals have skeletal involvement
05Craniofacial

Subtle facial features

Some affected individuals share mild, non-distinctive facial features — not specific enough to suggest the diagnosis on their own.

  • Mild facial dysmorphism described in some cases
  • No consistent "DLL1 face"
  • Microcephaly in a subset
  • Macrocephaly reported less commonly
06Other systems

Other reported findings

Notch signaling participates in many tissues, so occasional involvement of other systems has been described — though less consistently.

  • Feeding difficulties in infancy
  • Gastrointestinal issues in some individuals
  • Vision or hearing differences (variable)
  • Cardiac findings described rarely; may relate to specific deletions

The shape of the spectrum

Two people with the same DLL1 variant can present quite differently. "Variable expressivity" is the textbook phrase. In practice, every family’s story is shaped by which features expressed, how loudly, in which combinations — and how early support arrived.

  • Mild — learning difference only
  • Moderate — developmental delay, autism, no seizures
  • Significant — intellectual disability with vertebral involvement
  • Severe — multiple systems affected

What we don’t yet know

  • Why such variability? Two individuals with similar variants can present quite differently. Genetic modifiers, environment, and developmental chance all likely contribute — relative weight unknown.
  • Is there a natural history? The disorder was identified in 2019; the oldest known affected individuals have not yet been followed for decades.
  • How many are undiagnosed? A disorder this new is almost certainly under-diagnosed. Older negative results may carry undetected variants awaiting reanalysis.

When clinicians should consider DLL1 testing

  1. Unexplained global developmental delay or intellectual disability, particularly with one or more features below.
  2. Autism spectrum disorder or autistic features alongside structural neurological or skeletal findings.
  3. Vertebral malformations — hemivertebrae, butterfly or fused vertebrae, congenital scoliosis — in a child with neurodevelopmental concerns.
  4. Non-specific MRI brain findings (thin corpus callosum, mild ventriculomegaly, white-matter changes) with developmental delay.
  5. Older negative genetic results (panel or trio exome predating 2019) in an overlapping phenotype — consider reanalysis or whole-exome resequencing.
  6. A 6q27 deletion on chromosomal microarray, even small, with overlapping clinical features — DLL1 may be among the deleted genes.

Next: how the diagnosis is made

A diagnosis is the beginning, not the end. The path from clinical suspicion to a confirmed DLL1 result has its own logic — see Diagnosis.

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