1. College of Animal Science and Technology, Northwest A&F University, Yangling, P.R. China;
2. National Key Laboratory of Biotherapy, Sichuan University, Chengdu, P.R. China;
3. Center for Stem Cell Biology and Tissue Engineering, Key Laboratory for Stem Cells and Tissue Engineering, Ministry of Education, Sun Yat-Sen University, Guangzhou, P.R. China;
4. National Swine Foundation Seed Farm of Ankang Yangchen Modern Agriculture Group Co. Ltd, Ankang, P.R. China