[1] Univ Calif Berkeley, Dept Mol & Cell Biol, Berkeley, CA 94720 USA
[2] Salk Inst Biol Studies, La Jolla, CA 92037 USA
[3] Univ Calif San Diego, Div Biol Sci, La Jolla, CA 92093 USA
[4] Univ Calif Santa Cruz, Santa Cruz Inst Particle Phys, Santa Cruz, CA 95064 USA