Hereditary vitamin D resistant rickets has been associated with a number of mutations within the DNA and ligand binding domains of vitamin D receptors (VDR). The aim of our study was to identify and characterize the causative mutations in three kindreds with this condition. Resistance of 1,25(OH)2D3 was confirmed in cultured skin fibroblasts in which there was no induction of 24-hydroxylase activity; binding of 1,25(OH)2D3 to VDR was undetectable in patients 1 and 2, but normal in patients 3 and 4. The coding region of the VDR gene was sequenced to seek mutations. A mutation in the VDR gene of patient 1 resulted in a STOP codon, patient 2 showed a 56 bp deletion leading to frameshift and premature termination of VDR; a point mutation of A to C lying within the hormone-binding domain was shown for patients 3 and 4, who were siblings. Transactivation studies confirmed that these were functional mutations. Gel shift assays using nuclear extract from patient 3 demonstrated that the mutation that altered a conserved amino acid (glutamine-259) known to be involved in heterodimerization with other nuclear receptors affected protein: protein interactions.