Vitamin D–dependent rickets type I (VDDR-I), also known as pseudo–vitamin D–deficiency rickets, appears to result from deficiency of renal vitamin D 1α-hydroxylase activity. Prior work has shown that the affected gene lies on 12q13.3. We recently cloned the cDNA and gene for this enzyme, mitochondrial P450c1α, and we and others have found mutations in its gene in a few patients. To determine whether all patients with VDDR-I have mutations in P450c1α, we have analyzed the P450c1α gene in 19 individuals from 17 families representing various ethnic groups. The whole gene was PCR amplified and subjected to direct sequencing; candidate mutations were confirmed by repeat PCR of the relevant exon from genomic DNA from the patients and their parents. Microsatellite haplotyping with the markers D12S90, D12S305, and D12S104 was also done in all families. All patients had P450c1α mutations on both alleles. In the French Canadian population, among whom VDDR-I is common, 9 of 10 alleles bore the haplotype 4-7-1 and carried the mutation 958ΔG. This haplotype and mutation were also seen in two other families and are easily identified because the mutation ablates a TaiI/MaeII site. Six families of widely divergent ethnic backgrounds carried a 7-bp duplication in association with four different microsatellite haplotypes, indicating a mutational hot spot. We found 14 different mutations, including 7 amino acid replacement mutations. When these missense mutations were analyzed by expressing the mutant enzyme in mouse Leydig MA-10 cells and assaying 1α-hydroxylase activity, none retained detectable 1α-hydroxylase activity. These studies show that most if not all patients with VDDR-I have severe mutations in P450c1α, and hence the disease should be referred to as "1α-hydroxylase deficiency."