MMP9 and esophageal squamous cell carcinoma: Metformin has been described to inhibit the expression of several members of the MMP family in cancer, such as MMP11 in male germ tumor cells [55], MMP9 in ovarian cancer cells and in fibrosarcoma cells [56,57], MMP2 in cholangiocarcinoma cells [58], melanoma cells [59] and endometrial carcinoma cells [60], and MMP2 and MMP9 in breast cancer cells [61] and in esophageal squamous cell carcinoma [62].