1. A. B. Malmberg and A. I. Basbaum, Departments of Anatomy and Physiology and W. M. Keck Foundation Center for Integrative Neuroscience, University of California, San Francisco, CA 92143, USA.
2. C. Chen and S. Tonegawa, Howard Hughes Medical Institute, Center for Cancer Research, and Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.