1. College of Animal Science and Technology, Northwest Agriculture and Forestry University, Yangling, Shaanxi, People's Republic of China
2. Department of Obstetrics, Gynecology and Reproductive Sciences, Center for Reproductive Sciences, The Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California San Francisco (UCSF) Medical Center, UCSF, San Francisco, California