1. Department of Human Genetics and Development, College of Physicians and Surgeons, Columbia University, New York 10032
2. Laboratory of Molecular Biology, University of Nymegen, Nymegen 6525 ED, Netherlands
3. Department of Medical Parasitology, University of Nymegen, Nymegen 6500 HB
4. Department of Immunology, Institut Pasteur, Paris, France