[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 Turin, AO San Luigi Gonzaga, Inst Sci, Cavalieri Ottolenghi Fdn, I-10043 Orbassano, TO, Italy