We thank J. Nathans, S. Hattar, N. Gaiano, P. Achanta, C. Montojo, D. McClellan, T. Shimogori, T. Moran, E. Newman, M. Taylor and W. Yap for comments on the manuscript. We also thank M. Bonaguidi, C. Montojo, J. Reyes, M. Armour, E. Velarde, N. Forbes-McBean, W.F. Han and the Johns Hopkins School of Medicine Microscope Facility for technical advice and assistance. This work was supported by US National Institutes of Health grant F31 NS063550 and an NSF Graduate Fellowship (to D.A.L.), a Basil O'Connor Starter Scholar Award and grants from the Klingenstein Fund and NARSAD (to S.B.). S.B. is a W.M. Keck Distinguished Young Scholar in Medical Research.