1. State Key Laboratory of Reproductive Biology, Institute of Zoology (C.-G.L., Q.-Y.S.), Beijing 100080, China;
2. Graduate School (C.-G.L.), Chinese Academy of Sciences, Beijing 100080, China;
3. The Jackson Laboratory (Y.-Q.S.), Bar Harbor, Maine 04609;
4. Department of Molecular and Cellular Biology (H.-Y.F.), Baylor College of Medicine, Houston, Texas 77030;
5. Department of Veterinary Pathobiology (H.S.), University of Missouri-Columbia, Columbia, Missouri 65211