1. Department of Human Genetics and Development, and Department of Obstetrics and Gynecology, College of Physicians and Surgeons, Columbia University, New York 10032, U.S.A.
2. M.R.C Clincal Population Genetics Unit, Headington, Oxford, England; Present address: Department of Genetics, University of Leiden, Leiden, The Netherlands
3. Department of Tumor Biology, Karolinska Institutet, S-104 01 Stockholm 60, Sweden
4. Sir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, England