Association between MTHFR c.677C>T variant and erectile dysfunction among males attending fertility clinic oleh Shun Bai, Ming-Zhen Li, Yang-Yang Wan, Xue-Chun Hu, Yi-Xun Liu, Xian-Hong Tong, Tong-Hang Guo, Lu Zong, Ran Liu, Yuan-Qi Zhao, Ping Xiang, Bo Xu, Xiao-Hua Jiang