SLC5A5 and thyroid cancer: Structurally, NIS is a 13-transmembrane segment glycoprotein with an extracellular amino-terminus and a large intracellular carboxy-terminus, which contains a conserved monoleucine-based sorting motif that is required for NIS basolateral plasma membrane expression in the thyroid follicular cell (8, 9), and involved in reduced NIS plasma membrane expression in thyroid cancer (10).