Trophinin mediates homophilic and apical cell adhesion between trophoblastic cells and endometrial epithelial cells, which is potentially the initial attachment step in human embryo implantation. Since trophinin is an atypical membrane protein without the signal sequence, it is possible that trophinin localizes to the cytoplasm. By treating trophinin-expressing trophoblastic cells with a series of detergents, we found significant levels of endogenous trophinin in the cytoplasm, particularly at the nuclear envelope (NE). Fluorescence photobleaching of GFP-trophinin expressed in COS-1 cells showed the stable association of trophinin with the NE, suggesting an additional role of trophinin besides apical cell adhesion.