SLC5A5 and thyroid tumor: A major drop in NIS transcripts has been demonstrated in primary and metastatic thyroid tumors by comparison with normal tissues, but this is far less evident in metastases with no radioiodine (131I) uptake than in primary cancers and metastases able to trap 131I, suggesting that mechanisms other than a mere genetic control over NIS transcription might be involved in this failure to trap 131I [5].