These findings suggest that because RET/PTC rearrangements are not present in a majority of tumor cells, either a fraction of post-Chernobyl papillary thyroid tumors are of multiclonal origin, or ret rearrangement is a later, subclonal event..     ///    Heterogeneity in the distribution of RET/PTC rearrangements within individual post-Chernobyl papillary thyroid carcinomas.. 