*Correspondence:
Zhi Zheng, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences, School of Basic Medicine, Peking Union Medical College, 5 Dongdan Santiao Beijing, 100005 P. R. China
Tel: +86-010-69156961
E-mail: zhizheng100@126.com,
Jifei Yang, Lanzhou Veterinary Research Institute, Chinese Academy of Agricultural Sciences, Xujiaping 1, Lanzhou, Gansu, 730046, P. R. China
Tel: +86-931-8342671
E-mail: yangjifei@caas.cn