Furthermore, others have observed RAS and Rb mutations in 40–50% of follicular adenomas and carcinomas (Suarez et al, 1990; Capella et al, 1996; Zou et al, 1998) and a downregulation of p27KIP1, which was correlated with tumoral progression (Erickson et al, 2000; Troncone et al, 2000). This evidence concerns the gene RB1 and follicular thyroid adenoma.