LI Zi-jian, Institute of Vascular Medicine, Peking University Third Hospital and Key Laboratory of Cardiovascular Molecular Biology and Regulatory Peptide, Ministry of Health, Beijing 100191, China (Tel: 86-10-82265519. Email: lzjgy1995@ 163.com) Correspondence to: Dr. GENG Li, Department of Gynecology and Obstetrics, Peking University Third Hospital, Beijing 100191, China (Email: gengli57@163.com)