Cancer cells often exhibit loss of functional tight junctions (TJ), and disruption of the TJ structure is associated with cancer development. However, whether loss of a certain type of claudin, an integral membrane protein of TJ, is involved in malignant phenotypes remains to be clarified. Based on a report that claudin‐6 functions as a tumor suppressor for breast cancer, the authors show here that suppression of claudin‐6 expression results in increased resistance to various apoptogens, and causally enhances anchorage‐independent growth properties. Because claudin‐6 expression is partially silenced by promoter CpG island hypermethylation in MCF7 breast carcinoma cells, a synergistic effect of a demethylator and histone deacetylase inhibitor up‐regulates the expression of endogenous claudin‐6, which is sufficient for apoptotic sensitization and abrogation of colony‐forming efficacy. In addition, decreased expression of claudin‐6 promotes cellular invasiveness and transendothelial migration, accompanied by an increase in matrix metalloproteinase activity. These data suggest that the methylator phenotype of claudin‐6 may at least partially contribute to enhanced tumorigenic and invasive properties of breast carcinoma cells. (Cancer Sci 2007; 98: 1557–1562)