SLC5A5 and thyroid cancer: In humans, the rs965513[A] allele, which confers the greatest relative risk for the development of thyroid cancer, is located within an enhancer element controlling expression of FOXE1. FOXE1 is a thyroid-specific transcription factor that regulates several genes involved in thyroid hormone production, including TPO, thyroglobulin (TG), sodium-iodide symporter (SLC5A5), and dual oxidase (DUOX2).