1. Developmental Genetics Program, Skirball Institute of Biomolecular Medicine, and Department of Cell Biology, New York University School of Medicine, New York, NY 10016, USA.
2. Department of Molecular and Cellular Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA.
3. Department of Biology, Graduate School of Science and Technology, Kobe University, 1-1 Rokkodaicho, Nadaku, Kobe 657-8501, Japan.
4. Wellcome Trust Sanger Institute, Hinxton, Cambridge CB10 1SA, UK.