SummaryOBJECTIVE We wished to determine the abnormality responsible for Generalired Resistance to Thyroid Hormone in a family with this syndrome.DESIGN Molecular biological studies were performed on a mutant human thyroid hormone receptor beta (hTRβ) cloned from fibroblasts of the patient.PATIENTS The patient is from a previously reported family with typical features of Generalized Resistance to Thyroid Hormone, demonstrating goitre, elevated thyroid hormone levels, slightly elevated TSH, and retarded bone age.MEASUREMENTS A cDNA for hTRβ1 was cloned using specific oligonucleotide primers from fibroblast DNA. A mutant hTRβ1 expression vector was constructed, and an in‐vitro expressed mutant receptor was tested for T3 binding. Receptor binding to DNA was studied in a DNA cellulose assay and gel mobility shift assay.RESULTS Two mutations were found in the cloned hTRβ. One was silent but the second changed arginine 438 to histidine. The mutation was present in RNA and genomic DNA, as shown by allele‐specific amplification. The mutated receptor had reduced T3 binding affinity but demonstrated normal binding in a DNA cellulose assay and In a gel mobility shift assay. The receptor did not have altered heat sensitivity.CONCLUSIONS In the T sibship with Generalized Resistance to Thyroid Hormone, resistance to thyroid hormone is apparently produced by a substitution of a histidine for arginine at amino acid 438, which causes reduced binding of receptor to T3, although the receptor remains able to bind to DNA and, for this reason, functions as a dominant negative in affected subjects who are heterorygous with one normal and one mutated