Introduction Hereditary 1,25-dihydroxyvitamin D3 I1,25(OH)2D31 resistant rickets (HVDRR) is an autosomal recessive disease caused by target organ resistance to the action of 1,25(OH)1D3, the active form of the hormone. The defect in target cells is heterogenous and commonly appears to be a mutation in the gene encoding the vitamin D receptor (VDR). We have studied cultured skin fibroblasts and Epstein-Barr virus transformed lymphoblasts of seven family branches of an extended kindred having eight children affected with HVDRR. We have previously shown that cells from three affected children in this group contain an "ochre" nonsense mutation coding for a premature stop codon in exon 7 within the steroid-binding domain of the VDR gene.In The human VDR is a 50-kD protein and the cDNA has recently been cloned and sequenced (17) and found to belong to the steroid-thyroid-retinoic acid receptor superfamily of genes (18). Like the other members of this receptor family, VDR is comprised of at least two functional domains, a steroid hormone-binding domain and a DNA-binding domain (17-23). The steroid-binding domain stretches from amino acid 182 to the COOH-terminus at amino acid 427 and provides the molecule with high affinity, steroid-specific recognition of the 1,25(OH)2D3 metabolite. The DNA-binding domain, comprising amino acids 24-89, is a cysteine-rich region that contains two putative "zinc-fingers" (17). 1,25(OH)2D3 binding to the steroid-binding domain of the receptor "activates" the VDR to a form with high affinity for DNA. The DNA-binding domain of the VDR then binds to a vitamin D response element in the regulatory region of target genes to modulate transcription of responsive genes and thereby mediates hormone action (17)(18)(19)(20)(21)(22)(23).Progress to elucidate the molecular basis of HVDRR began with the demonstration that rodent skin possessed VDR (24, 25) and that cultured human dermal fibroblasts derived from skin biopsies could be used as a model system for the study of the VDR from patients (26). It was soon demonstrated that defects in the VDR were the likely cause of the HVDRR syndrome (5) and that different families exhibited different defects (5-16). An additional useful finding was the demonstration that 1,25(OH)2D3 could induce the enzyme 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase) in multiple target tissues by a receptor-mediated process (27). It was subsequently found that 24-hydroxylase activity could be induced in cultured skin fibroblasts so that induction of 24-hydroxylase activity could be employed as a bioassay ofthe ability of cultured fibroblasts to respond to 1,25(OH)2D3 (5). Using this bioresponse marker, cells from a variety of patients with HVDRR