<prompt> Answer this question truthfullyWhat type of cells express the RET proto-oncogene, and in what syndromes are activating mutations of this gene seen? Specifically, which cancer is associated with these mutations?  <response>: <reason> The RET proto-oncogene is expressed by cells of neural crest lineage, and activating mutations are seen in multiple endocrine neoplasia MEN II syndromes, specifically medullary carcinoma of the thyroid. 