1. Shandong Provincial Key Laboratory of Animal Disease Control and Breeding, Institute of Animal Science and Veterinary Medicine, Shandong Academy of Agricultural Sciences, Jinan, Shandong 250100, China
2. Department of Animal Sciences, D.H. Barron Reproductive and Perinatal Biology Research Program, and Genetics Institute, University of Florida, Gainesville, FL 32611-0910, USA
3. Department of Animal Science, University of California, Davis, CA 95616, USA
4. Stem Cell Core, Salk Institute for Biological Studies, La Jolla, CA 92037, USA