Correspondence to: LI Lan-ying, 2011 Collaborative Innovation Center of Tianjin for Medical Epigenetics, Key Laboratory of Hormones and Development (Ministry of Health), Metabolic Diseases Hospital & Tianjin Institute of Endocrinology, Tianjin Medical University, Tianjin 300070, China (Tel: 86-22-23373728. Email: lily@tijmu.edu.cn)