1. Z. Xia and M. E. Greenberg are in the Division of Neuroscience, Department of Neurology, Children's Hospital, and Department of Neurobiology, Harvard Medical School, Boston, MA 02115, USA.
2. M. Dickens, J. Raingeaud, and R. J. Davis are in the Howard Hughes Medical Institute and Program in Molecular Medicine, Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, Worcester, MA 01605, USA.