Salivary glands do express hNIS protein, as shown by immunohistochemical studies using a specific antihNIS antibody (21, 26). Immunostaining of the NIS protein occurs only in the ductal cells, where it is diffusely expressed within cells, and not in the acinar cells (Fig. 4). In the human stomach, expression of NIS was restricted to parietal cells. Recently, characterization of rat gastric NIS by Western blot analysis showed the presence of a small amount of immature gastric NIS, with different molecular weights compared with the thyroid protein (27). This suggests the existence of tissue-specific post-translational mechanisms in the functional regulation of NIS expression.