Nucleotide sequence analysis of PCR fragments of TSH receptor cDNA synthesized from thyroid RNA of a patient with autoimmune hyperthyroidism, revealed two different sequences in the first position of codon 36. In one of the sequences, there was a C for G substitution leading to the D36----H substitution in the predicted peptide. Both variants were also found in genomic DNA of thyroid tissue. However, only the germ line sequence was found in other tissues representing all three different germ layers. The novel sequence is therefore likely to represent a somatic mutation in the thyroid tissue, of possible relevance for the pathogenesis of the patient's thyroid disorder.