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Condition report · Neurological

Nizon-Isidor Syndrome

A rare neurodevelopmental condition causing developmental delay, intellectual disability and speech difficulties, sometimes with autism or other behavioural differences. It is caused by a change in one copy of MED12L.

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The condition we analyse

Autosomal dominant

Caused by a change in one copy of MED12L, which encodes part of the Mediator complex that helps control gene activity. Reported changes were earlier thought to arise de novo, but a maternally inherited case has now been described.

Signs and symptoms

Developmental delay and intellectual disability, with speech impairment. Autism and other behavioural differences have been reported. Less often there are differences of the corpus callosum on brain scans or mild facial features.

What a diagnosis changes

Developmental, speech, educational and behavioural support. Changes can arise de novo or be inherited from a parent, so genetic counselling and parental testing guide recurrence risk.

Genes analysed

Also known as

NIZIDS

MONDO:0030030OMIM 618872MedGen 1715748Sources: [1][2][3]

References

  1. Nizon M, Laugel V, Flanigan KM, Pastore M, Waldrop MA, Rosenfeld JA et al., 2020. Variants in MED12L, encoding a subunit of the mediator kinase module, are responsible for intellectual disability associated with transcriptional defect. Genetics in medicine : official journal of the American College of Medical Genetics
  2. Stewart R, Ezell KM, Bell DS, Corner B, McMinn A, Cogan JD et al., 2025. Case Series of Nizon-Isidor Syndrome by Heterozygous Variants in MED12L With Further Evidence of Mitotic Instability in One Case With Diploid-Triploid Mosaicism. American journal of medical genetics. Part A
  3. Dutta D, Black J, Macaya D, McGivern B, Garg R, 2026. Evidence of maternal inheritance of Nizon-Isidor syndrome in an individual with GAMT and TNFRSF13B sequence variants. Journal of human genetics

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