Vol. 190 No. 3, August 9, 2010. Pages 407–425.

Due to a production error, a source of funding was omitted from the authors' acknowledgments. The grant information has been restored in the html and pdf versions of the paper, and the error remains only in print. The corrected acknowledgment paragraph appears below.

This work was supported by the Fonds de la Recherche en Santé du Québec (to S. Jean), the Kimmel Foundation (grant KF06-120 to A.A. Kiger), the Packard Foundation (grant 2005-29096 to A.A. Kiger), and the National Institutes of Health (grants R01 GM078176 to A.A. Kiger, RO1 DK17780 to M. Farquhar and the CMM Electron Microscope Facility, and P30 NS047101 to the University of California, San Diego Neuroscience Microscopy Shared Imaging Facility).