1. J. Dodd is in the Department of Physiology and Cellular Biophysics and Center for Neurobiology and Behavior, Columbia University, New York, NY 10032, USA.
2. T. M. Jessell is at the Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY 10032, USA..
3. M. Placzek is in the Developmental Genetics Programme, Krebs Institute, University of Sheffield, Sheffield S10 2TN, UK.