1. O. A. Ryder is at the Center for Reproduction of Endangered Species, Zoological Society of San Diego, CA 92112-0551, USA.
2. A. McLaren is at Wellcome/CRC Institute, University of Cambridge, CB2 1QR, UK.
3. S. Brenner is at the Molecular Sciences Institute, Berkeley, CA 94704, USA.
4. Y.-P. Zhang is at the Key Laboratory of Cellular and Molecular Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, China.
5. K. Benirschke is at the Department of Pathology, University of California, San Diego, CA 92093, USA.