This study investigated the effects of estradiol (E2) and 4-hydroxytamoxifen (4-OHT) on the expression of the tight junction protein occludin and the invasive capability of two endometrial adenocarcinoma cell lines, HEC-1A and HEC-1B. The results showed that E2 had a biphasic effect on occludin expression, initially increasing the expression of lower molecular weight isoforms at 10nM in both cell lines. 4-OHT inhibited occludin expression in a dose-dependent manner. E2 also decreased the invasive capability of both cell lines in a dose-dependent manner. These findings suggest that low concentrations