By real-time quantitative RT-PCR, high levels of TG mRNA were expressed in some patients without evident metastasis. For example, in one patient, the copy number of TG mRNA in 1 ml of peripheral blood was comparable to that in over 300 pg total RNA from normal thyroid tissue, which is approximately equal to 30 thyroid cells. It is hard to believe that this many thyroid cells still circulated in the peripheral blood of a patient after thyroidectomy without any evidence of a distant metastasis, which could be a source of circulating cells. TSH receptor mRNA, which has previously been considered to be expressed only in the thyroid, was found to be expressed in adipocytes and lymphocytes (Francis et al, 1991; Endo et al, 1995) . The TG mRNA detected by our study thus might not have been derived from thyroid cells; instead, it is most likely that lymphocytes express small quantities of TG mRNA, as they are known to express various kinds of mRNAs such as alpha-fetoprotein (AFP) or carcinoembryonic antigen (CEA) (Lafarge-Frayssinet et al, 1989; Coutelier et al, 1994) , which used to be considered tumour-specific. Further, the expression of TG mRNA is not likely to be limited to thyroid follicular cells, as we have recently found it to be expressed in cell lines derived from lung or gastric carcinomas (data not shown). Possibly, the regulation of TG mRNA expression in lymphocytes is controlled by some unknown factors. If so, the detection of TG mRNA is not likely to become an alternative means of achieving early detection of recurrent thyroid carcinomas because sensitive detection of recurrence would be interfered with by basal expression of TG mRNA in peripheral blood.