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The absence of a piece of DNA containing just 17 genes can cause a huge range of birth abnormalities, including heart defects, immune-system deficiencies and cleft palate, writes Charles Arthur.

Adults who lack the same stretch of genetic material are also more likely to develop obsessive-compulsive disorders, such as excessive hand-washing, John Burn, professor of clinical genetics at Newcastle University, told the meeting.

The block of DNA responsible for the problems normally lies on the main part of chromosome 22, of the 23 pairs of human chromosomes. It is about 2 million base pairs long, and contains 17 gene locations. In people with problems, the genes "are known to be missing, but we don't know which is the important one", said Professor Burn. Studies are now under way to try to pin it down.