TPO and neoplasm: Downregulation of gene expression in TC is often related to epigenetic mechanisms, such as aberrant DNA methylation [66], which accounts for the downregulation of tumor suppressor genes such as RASSF1A (ras association domain family member 1), SLC5A8 (solute carrier family 5 member 8), RARβ2 (retinoic acid receptor beta 2), and RAP1GAP (rap1 GTPase-activating protein 1), as well as thyroid-specific genes, such as TSHR (thyroid stimulating hormone receptor) and TPO (thyroid peroxidase), and SLC5A5 (solute carrier family 5 member 5) also known as NIS [67].