TPO and thyroid cancer: In contrast, KTC-1 are functionally poorly differentiated papillary thyroid carcinoma cells that, despite not expressing TSH receptors, thyroid peroxidase (TPO) and sodium iodide symporter (NIS), retain high transcript levels of TG, TTF-1 and paired box gene 8 (PAX8) relative to other thyroid cancer cell lines [20,29,30].