1. Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA.
2. Department of Laboratory Medicine and Division of Medical Genetics (Medicine), University of Washington, Seattle, WA 98195–7110, USA.
3. The Jackson Laboratory, Bar Harbor, ME 04609, USA.
4. Harvard Medical School and Department of Neurology, Beth Israel Deaconess Medical Center, Boston, MA 02115, USA.