Pseudovitamin D-deficiency rickets (PDDR) is an autosomal recessive disorder that may be due to impaired activity of 25-hydroxyvitamin D-1α-hydroxylase, a renal cytochrome P450 enzyme (P450_1α) of the vitamin D pathway. The disease locus for PDDR has been mapped by linkage analysis to 12q13-q14, but the molecular defect underlying the enzyme dysfunction has remained elusive due to the lack of sequence information for the P450_1α gene (hereafter referred to as 1α-OHase). We have used a probe derived from the rat 25-hydroxyvitamin D-24-hydroxylase (CYP24; 24-OHase) sequence to identify and clone the 1α-OHase cDNA. The full-length 1α-OHase clone of 2.4 kb codes for a protein of predicted Mr 55 kDa. Functional activity of the cloned sequence was assessed using transient transfection, and the production of authentic 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃] was confirmed using high performance liquid chromatography fractionation and time-of-flight mass spectrometry. The expression of the gene was analyzed in vitamin D–replete animals; treatment with 1α,25(OH)₂D₃ reduced 1α-OHase transcript levels by 70%, while administration of parathyroid hormone led to a 2-fold increase in the expression of the gene, thus confirming the hormonal regulation previously described using biochemical methods. The rat cDNA was used to obtain a human genomic clone. Interestingly, the human 1α-OHase gene mapped to 12q13.1-q13.3, providing strong evidence that a mutation in the 1α-OHase gene is responsible for the PDDR phenotype. The availability of a cloned sequence for 1α-OHase generates novel tools for the study of the molecular etiology of PDDR, and will allow the investigation of other disturbances of vitamin D metabolism.