The precise molecular abnormalities that cause primary cortisol resistance have not been completely described. In a subject with primary cortisol resistance we have observed glucocorticoid receptors (hGR) with a decreased affinity for dexamethasone. We hypothesize that a mutation of the hGR glucocorticoid-binding domain is the cause of cortisol resistance. Total RNA isolated from the index subject's mononuclear leukocytes was used to produce first strand hGR cDNAs, and the entire hGR cDNA was amplified in segments and sequenced. At nucleotide 2,317 we identified a homozygous A for G point mutation that predicts an isoleucine (ATT) for valine (GTT) substitution at amino acid 729. When the wild-type hGR and hGR-Ile 729 were expressed in COS-1 cells and assayed for [3H]-Dexamethasone binding, the dissociation constants were 0.799 +/- 0.068 and 1.54 +/- 0.06 nM (mean +/- SEM) (P < 0.01), respectively. When the wild-type hGR and hGR-Ile 729 were expressed in CV-1 cells that were cotransfected with the mouse mammary tumor virus long terminal repeat fused to the chloramphenicol acetyl transferase (CAT) gene, the hGR-Ile 729 conferred a fourfold decrease in apparent potency on dexamethasone stimulation of CAT activity. The isoleucine for valine substitution at amino acid 729 impairs the function of the hGR and is the likely cause of primary cortisol resistance in this subject.