[1] Univ Geneva, Dept Genet & Microbiol, CH-1211 Geneva 4, Switzerland
[2] Salk Inst Biol Studies, La Jolla, CA 92037 USA
[3] Yeshiva Univ Albert Einstein Coll Med, Howard Hughes Med Inst, Bronx, NY 10461 USA
[4] Yeshiva Univ Albert Einstein Coll Med, Dept Mol Genet, Bronx, NY 10461 USA