1. Mental Retardation Research Center, University of California, Los Angeles, California 90095;
2. Department of Cell and Developmental Biology and
3. Vollum Institute, Oregon Health Sciences University, Portland, Oregon 97201;
4. Instituto de Investigaciones en Ingenierı́a Genética y Biologı́a Molecular, Consejo Nacional de Investigaciones Cientı́ficas y Técnicas and Departamento de Ciencias Biológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina; and
5. Department of Neuroscience, The Chicago Medical School, North Chicago, Illinois 60064