Different point mutations have been identified in the T₃-binding domain of the c-erbAβ thyroid hormone receptor gene that are associated with variant phenotypes of generalized thyroid hormone resistance (GTHR). In most cases of GTHR, heterozygotes are affected; a single mutant allele results in the inhibition of the function of normal thyroid hormone receptors. We report here a novel genetic abnormality, a 3-basepair (bp) deletion in the T₃-binding domain of the β-receptor in a kindred, S, with GTHR. One patient, S1, was the product of a consanguineous union of two heterozygotes and was homozygous for this defect. Heterozygotes from kindred S harbored a CAC deletion at nucleotides 1295–1297, which resulted in the deduced loss of amino acid residue threonine at codon 332, and they displayed elevated free T₄ levels and inappropriately normal TSH levels characteristic of other kindreds with GTHR. However, patient S1, who had two mutant alleles, had markedly elevated TSH and free T₄ levels and displayed profound abnormalities in brain development and linear growth. A fibroblast c-erbAβ cDNA extending from codon 175 to stop codon 457 was cloned from patient S1, sequenced, and used to create a full-length mutant cDNA. The kindred S mutant receptor was synthesized in vitro and did not bind T₃. This mutant receptor did bind with similar avidity as the wild-type human β-receptor to thyroid hormone response elements of the human TSHβ (−12 to 43 bp) and rat GH (−188 to −160 bp) genes. Kindred S showed the effect in man of heterozygous and homozygous expression of a dominant negative form of c-erbAβ.