[1] Yale Univ, Sch Med, Howard Hughes Med Inst, New Haven, CT 06510 USA
[2] Yale Univ, Sch Med, Dept Cell Biol, New Haven, CT 06510 USA
[3] Stanford Univ, Dept Mol & Cellular Physiol, Stanford, CA 94305 USA
[4] Stanford Univ, Dept Neurol & Neurol Sci, Stanford, CA 94305 USA
[5] Stanford Univ, Stanford Synchrotron Radiat Lab, Stanford, CA 94305 USA
[6] Sanger Ctr, Cambridge CB10 1SA, England