1. Department of Cell and Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK
2. Stowers Institute for Medical Research, Kansas City, MO 64110, USA
3. Department of Anatomy and Cell Biology, University of Kansas Medical Centre, Kansas City, KS 66160, USA
4. Division of Biology, California Institute of Technology, Pasadena, CA 91125, USA