[1] UCL, MRC, Mol Cell Biol Lab, London, England
[2] Yale Univ, Sch Med, Dept Cellular & Mol Physiol, New Haven, CT 06510 USA
[3] Univ Texas SW Med Ctr Dallas, Dept Dev Biol, Dallas, TX 75390 USA
[4] Univ Texas SW Med Ctr Dallas, Kent Waldrep Fdn Ctr Basic Res Nerve Growth & Reg, Dallas, TX 75390 USA