The syndrome of hereditary resistance to 1,25-dihydroxyvitamin D3 is due to defective function of the vitamin D receptor (VDR). The recent cloning and nucleotide sequence determination of the human VDR chromosomal gene have enabled a direct evaluation of the genetic basis for this disease in affected patients. In this report we employed polymerase chain reaction techniques to amplify the gene exons that encode the DNA-binding domain of the VDR from two 1,25-dihydroxyvitamin D3-resistant patients whose receptors displayed defective binding to nonspecific DNA. Although their families were apparently unrelated, each patient displayed an identical homozygous point mutation within the third exon, a mutation that causes substitution of a glutamine for an arginine residue highly conserved within the entire steroid receptor superfamily. We introduced this base change into the normal VDR cDNA via site-directed mutagenesis, transfected an expression vector containing this cDNA into cells, and examined the functional properties of the resultant VDR expression product. The produced mutant receptor bound 1,25-dihydroxyvitamin D3 with normal affinity, but displayed weak affinity for the nuclear fraction and for heterologous DNA. More importantly, the protein was inactive in promoting transcription in a cotransfection assay employing a chloramphenicol acetyltransferase gene reporter fused down-stream of the VDR-inducible osteocalcin gene promoter-enhancer. These results provide the genetic and functional basis for the phenotype of rickets in this inherited disease.