 RET/PTC oncogenes, generated by chromosomal rearrangements in papillary thyroid carcinomas, are constitutively activated versions of proto-RET, a gene coding for a receptor-type tyrosine kinase (TK) whose ligand is still unknown.     ///    RET/PTCs encode fusion proteins in which proto-RET TK and C-terminal domains are fused to different donor genes. 