[1] Trinity Coll Dublin, Sch Genet & Microbiol, Dublin 2, Ireland
[2] Trinity Coll Dublin, Sch Nat Sci, Smurfit Inst Genet, Dublin 2, Ireland
[3] Trinity Coll Dublin, Trinity Coll Inst Neurosci, Dublin 2, Ireland
[4] St Jude Childrens Res Hosp, Dept Dev Neurobiol, Memphis, TN 38120 USA
[5] Univ Colorado, Dept Biochem & Chem, Boulder, CO 80309 USA
[6] Univ Colorado, Howard Hughes Med Inst, Boulder, CO 80309 USA