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Severe intellectual disability (severe cognitive impairment, an IQ of <50) affects about one in 200–300 people in developed countries. For most children with nonsyndromic severe intellectual disability and no clear prenatal or postnatal insult no diagnosis is made. A genetic cause is often suspected but cannot be proved. De novo point mutations have been suggested as a likely cause and two papers published in 1 week in November 2012 have shown the importance of such mutations.
A study at 10 centres in Germany and Switzerland (Anita Rauch and colleagues. Lancet 2012;380:1674–82; see also Comment, ibid: 1630–1) included 51 case families (child with severe intellectual disability, cause unknown after extensive investigation, and two intellectually normal parents) and 20 control family trios from a type 2 diabetes family study. By comparing exome sequencing of …