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Publisher Summary The Na+/I- symporter (NIS) is a key plasma membrane protein that catalyzes the active accumulation of iodide (I-) in the thyroid gland, that is, the first and critical rate-limiting step in the biosynthesis of the thyroid hormones. NIS is located in the basolateral membrane of the hormone-producing thyroid follicular cells or thyrocytes. NIS is one of at least four major proteins that appear to be distinctively thyroidal, the other three being Tg, TPO and the TSH receptor. Each of these four proteins mediates a crucial step in thyroid hormogenesis. Each of them also plays an important role in thyroid function. They are also considered thyroid-specific markers. This chapter presents the rat NIS cDNA clone and discusses a secondary structure model for the NIS molecule. The product of the NIS cDNA clone is sufficient to elicit Na+/I- symport activity in both oocytes and mammalian cells, suggesting that NIS probably functions as a single subunit or as an oligomer of identical subunits. The role of NIS as the mediator of I- accumulation is consistent with its placing alongside other Na+-dependent anion transporters based on clustering relationships between its deduced amino acid sequence and the deduced sequences of other transporters. A comparison of the predicted amino acid sequence of NIS with those of other cloned Na+-dependent cotransporters revealed the highest degree of homology.