RET and medullary thyroid gland carcinoma: Mutations of the RET gene are usually observed in papillary and medullary carcinomas, which are key factors in tumorigenesis and progression.196,197 Somatic gene rearrangements and fusions of RET occur in ~2.5–73% of papillary thyroid carcinomas, which are the most common subtype of differentiated thyroid cancer.