[1] Univ Calif San Francisco, Dept Anat, San Francisco, CA 94158 USA
[2] CALTECH, Div Biol, Pasadena, CA 91125 USA
[3] CALTECH, Howard Hughes Med Inst, Pasadena, CA 91125 USA
[4] Univ N Carolina, Dept Cellular & Mol Physiol, Chapel Hill, NC 27599 USA