1. W.F. Goodman Center for Comparative Medical Genetics, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.
2. Stokes Institute, Children's Hospital of Philadelphia, Philadelphia, PA 19104.
3. Present address: Department of Neurosurgery, School of Medicine, University of Pennsylvania, PA 19104.
4. Department of Radiology, School of Medicine, University of Pennsylvania, Philadelphia, PA 19104.
5. Department of Radiology and Institute for Cellular Engineering, Johns Hopkins University School of Medicine, Baltimore, MD 21205.