RET and thyroid gland undifferentiated (anaplastic) carcinoma: Transgenic mice carrying TRK and RET/PTC3 oncogenes develop thyroid papillary carcinomas (Powell et al, 1998; Russell et al, 2000); thyroid anaplastic carcinomas were, conversely, obtained in the Tg-SV40 mice (Ledent et al, 1991).