1Wessex Regional Genetics Laboratory, Salisbury District Hospital, Salisbury, Wilts SP2 8BJ, UK Fax: +44-1722-338095, , GB
2Wessex Clinical Genetics Service, Princess Anne Hospital, Southampton, SO16 5YA, UK, , GB
3Behavioural Sciences Unit, Institute of Child Health, 30 Guilford Street, London WC1 1EH, UK, , GB