The antidiabetic drug metformin exerts chemopreventive and antineoplastic effects in many types of malignancies.However, the mechanisms responsible for metformin actions appear diverse and may differ in different types of cancer.Understanding the molecular and cellular mechanisms specific for different cancers is important to optimize strategy for metformin treatment in different cancer types.Here, we investigate the in vitro and in vivo effects of metformin on esophageal squamous cell carcinoma (ESCC) cells.Metformin selectively inhibited cell growth in ESCC tumor cells but not immortalized noncancerous esophageal epithelial cells.