 RET/PTC1 is a rearranged form of the RET proto-oncogene detected in human papillary thyroid carcinomas.     ///    "The roles of phosphotyrosines-294, -404, and -451 in RET/PTC1-induced thyroid tumor formation.. "    ///    "Signal transduction pathways mediated by phosphotyrosine 294, 404, or 451 in RET/PTC1 have been shown to be critical for RET-induced transforming activity in vitro. "    ///    "This indicates that signaling pathways mediated by pY294, pY404, and pY451 do play a role in RET/PTC1-induced tumor formation. "    ///    "However, as tumors are still able to form in some mice within these three mutant transgenic groups, it indicates that none of the signaling pathways mediated by pY294, pY404, or pY451, are solely essential for RET/PTC1-induced tumor formation.. "    ///    "To investigate the contribution of these signaling pathways in RET/PTC1-induced thyroid tumor formation in vivo, we generated and characterized transgenic mice expressing thyroid-targeted RET/PTC1 mutants carrying a site-directed mutation changing tyrosine (Y) to phenylalanine (F) at the residue 294, 404, or 451. "    ///    We previously showed that thyroid-targeted expression of RET/PTC1 leads to thyroid tumor formation in Tg-PTC1 transgenic mice. 