[1] Univ Calif San Francisco, Howard Hughes Med Inst, Dept Anat, San Francisco, CA 94143 USA
[2] Univ Calif San Francisco, Howard Hughes Med Inst, Dept Biochem & Biophys, San Francisco, CA 94143 USA
[3] Univ Calif Los Angeles, Howard Hughes Med Inst, Dept Biol Chem, MacDonald Res Labs 5748, Los Angeles, CA 90095 USA