Nature Genet. doi:10.1038/ng.534 (2010)

A chunk of DNA missing from chromosome 16 has been linked to intellectual disability.

Evan Eichler at the University of Washington in Seattle and his colleagues report a rare deletion of 520,000 base pairs from the chromosome. The deletion was present in 42 of 21,127 children with intellectual disability, but only 8 of 14,839 children of normal ability.

Some of the children had additional genetic defects, and were more severely disabled. This supports a 'two-hit' model that could explain why children with only the chromosome 16 deletion had a lower degree of impairment. Further analysis of different genetic defects suggests that this two-hit model extends to other neuropsychiatric conditions.