RASSF1 and parathyroid gland adenoma: The frequently observed hypermethylation of the RASSF1A promoter and downregulation of RASSF1A mRNA levels in parathyroid adenomas point toward an association between the inactivation of this tumour suppressor gene and parathyroid aberrant growth, whereas APC gene expression is retained in parathyroid adenomas due to increased transcription from APC promoter 1B.