Expression of the RET/PTC1 oncogene impairs the activity of TTF-1 and Pax-8 thyroid transcription factors..     ///    "We conclude that the RET/PTC1 oncogene alters the expression of the thyroid-differentiated phenotype by at least two different mechanisms, ie., down-regulation of Pax-8 protein and mRNA expression and impaired function of TTF-1 and Pax-8, which occurs at a posttranslational level.. "    ///    We recently found that the expression of the RET/PTC1 oncogene blocked the expression of the thyroid-differentiated phenotype in rat thyroid epithelial cell line PC CI 3 (PC).     ///    " The most frequent genetic alterations described thus far in human papillary thyroid carcinomas are somatic rearrangements of the RET proto-oncogene, which generate the chimeric RET/PTC oncogenes. "