Kleefstra syndrome (Ks) is a rare genetic disorder characterised by intellectual disability, often accompanied by a spectrum of complex physical and clinical features. The predominant cause of Ks is a tiny piece missing (known as a deletion) from near the end of chromosome 9. However individuals with a mutation, or intragenic duplication also carry a Ks diagnosis. The deletion or mutation affects a gene called EHMT1 (Euchromatic Histone Methyltransferase 1) and it’s absence or disturbance is believed to cause the major symptoms of the syndrome. The syndrome was officially recognised as Kleefstra syndrome in April 2010. Ks has previously been known as 9q34.3 deletion syndrome, 9qSTDS (short for 9q subtelomeric deletion syndrome), 9q-Syndrome, 9q34.3 microdeletion syndrome.

There isn’t a one-size-fits-all description of Ks because there is a wide range of symptoms and an even wider range in the severity of those symptoms. The variation may be due in part to the number of damaged or deleted genes in the 9q34.3 region, but those with roughly the same deletion size can also have quite different symptoms.

Most cases of Ks are de novo meaning they are not inherited from either parent, however even though rare it has been known for a child to inherit the 9q34.3 deletion from an unaffected parent who is mosiac for the deletion. Mosiaic means that an individual has the deletion is some cells but not in others. Also very rarely individuals with Ks have been known to reproduce and pass the disorder onto their children.