Hyperfunctioning thyroid adenomas are clonal neoplasms with the intrinsic capacity of growing and differentiate independently of thyroid-stimulating hormone (TSH). We analysed the mRNA encoding thyrotropin receptor of 11 adenomas obtained by fine needle aspiration biopsy (FNAB) and found 7 mutants all located in three aminoacids clustered in the sixth transmembrane domain of the receptor. These mutations were somatic and specifically present in the tumour tissue. DNA sequence revealed that 80 to 90% of the mutations can be rapidly screened and identified by restriction enzyme analysis of the amplified cDNA obtained from the FNABs. The mutation Thr->Ile was introduced in the wild type receptor and expressed in mouse fibroblasts. These cells constitutively activate the transcription of a reporter gene under the control of cyclic AMP responsive element.