(b) p18 and p27. Thyroid tumors show low expression of the cyclin-dependent kinase inhibitor (CDKI) p27 (Kip1), and recent evidence demonstrates that p27 is downregulated by the active RET mutant, RET/PTC1, which is found in papillary thyroid carcinomas. This evidence concerns the gene RET and thyroid gland papillary carcinoma.