1. Division of Gene Function and Development, Roslin Institute (Edinburgh), Roslin, EH25 9PS, UK.
2. Present address: Allergy Research Centre, 2nd Paediatric Clinic,University of Athens, 41 Fidipidou Street,Athens 11527,Greece.
3. Present address: Centre for Regenerative Medicine,Chancellor's Building, Room GU507,49 Little France Crescent, Edinburgh, EH16 4SB, UK.