1. Karolinska Institutet, Department of Biosciences Novum/University College Sodertorn, Department of Natural Sciences, S-141 04 Huddinge, Sweden;
2. 3-D Laboratory for Fine Structure, University of Colorado, Boulder, Colorado 80309;
3. Medical Research Council Human Genetics Unit, Edinburgh, United Kingdom EH4 2XU; and
4. Huddinge Sjukhus, Clinical Research Center, Electron Microscopy Unit, S-141 86 Huddinge, Sweden