This domain family is found in eukaryotes, and is approximately 180 amino acids in length. There is a single completely conserved residue P that may be functionally important. Symplekn has been localized, by light and electron microscopy, to the plaque associated with the cytoplasmic face of the tight junction-containing zone (zonula occludens) of polar epithelial cells and of Sertoli cells of testis. However, both the mRNA and the protein can also be detected in a wide range of cell types that do not form tight junctions. Careful analyses have revealed that the protein occurs in all these diverse cells in the nucleoplasm, and only in those cells forming tight junctions is it recruited, partly but specifically, to the plaque structure of the zonula occludens.