The RET proto-oncogene has been demonstrated to be expressed in medullary thyroid carcinomas and pheochromocytomas, and was mutated in patients with the multiple endocrine neoplasia type 2A (MEN 2A). The results presented herein show its expression in normal human thyroid parafollicular C cells. Since RET is predicted to encode a receptor for a still unknown ligand, these data confirm its involvement in the regulation and growth of these cells.