Mutations in the T3-binding domain of the thyroid hormone receptor gene c-erbA beta result in dominant negative proteins and thyroid hormone resistance syndromes. Variable clinical manifestations of resistance to thyroid hormones have been reported, including short stature and neuropsychological abnormalities. The molecular bases for heterogeneity of phenotype among and within kindreds have not been fully elucidated. Recent investigations have considered differential expression of mutant and wild-type beta 1-receptor alleles and the regulation thereof as a mechanism to explain differential sensitivity to thyroid hormones. We used reverse transcription-competitive polymerase chain reaction (PCR) to measure c-erbA beta 1, c-erbA alpha 1, and c-erbA alpha 2 mRNAs in skin fibroblasts cultured from normal subjects, heterozygotes, and a severely affected homozygous mutant of kindred S. The homozygous mutant of kindred S had severe growth and mental retardation. After reverse transcription with primers specific for each of the c-erbA mRNAs, first strand cDNAs were amplified by PCR using subtype-specific amplimers. Primer design allowed simultaneous detection of wild-type and mutant messages in heterozygous fibroblasts and showed an approximately 1:1 ratio of these mRNAs in three patients. Inclusion of competitive standard cDNAs of known concentration in the PCR reactions allowed quantitation of the absolute levels of the beta 1-, alpha 1-, and alpha 2 mRNAs by comparison of products on ethidium bromide-stained agarose gels. These studies showed no effect of the presence of the mutant beta 1-allele, as fibroblast RNA from normal subjects, heterozygotes, and the homozygote gave values of 56-184, 2.8-12, and 23-40 attomol/5 micrograms total RNA for beta 1-, alpha 1-, and alpha 2 mRNAs, respectively. We conclude that these sensitive methods allow the detection of molecular species present at levels as low as 10 molecules/cell, and that this potent dominant negative receptor does not disrupt c-erbA expression at the level of mRNA. The neuropsychological sequelae of the kindred S mutation are not due to relative overexpression of the mutant allele.