1. T. K. Hensch, M. Fagiolini, N. Mataga, Laboratory for Neuronal Circuit Development, Brain Science Institute RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan. M. P. Stryker, Department of Physiology, University of California, San Francisco, CA 94143, USA. S. Baekkeskov and S. F. Kash, Department of Medicine and Microbiology/Immunology, Hormone Research Institute, University of California, San Francisco, CA 94143, USA.