This study demonstrates that all thyroid carcinomas harboring activating RET rearrangements exhibit a well-differentiated phenotype, that of papillary carcinoma, and indicates that the subset of RET/PTC-positive papillary carcinomas do not progress to more aggressive, less differentiated tumor phenotypes..     ///    RET/PTC oncogene activation defines a subset of papillary thyroid carcinomas lacking evidence of progression to poorly differentiated or undifferentiated tumor phenotypes..     ///    "A common genetic alteration in thyroid tumors is the rearrangement of the tyrosine kinase-encoding RET proto-oncogene, leading to the generation of chimeric RET/PTC oncogenes. "